Savory Mushroom Onion Tart

There's something about the smell of onions turning golden in a hot pan that makes a kitchen feel like home. I stumbled onto this tart during one of those evenings when I had more vegetables than time, and somehow the combination of caramelized onions, earthiness from mushrooms, and that creamy, cheese-laden custard filling felt like discovering something I'd been making my whole life. The flaky pastry case just sealed the deal—it's one of those dishes that looks far more complicated than it actually is.

I made this for a small dinner party last autumn, and I remember one guest asking halfway through if I'd been to culinary school because they couldn't believe something so elegant had come from my oven. The truth was just time and a good tart pan, but that moment made me realize how often we underestimate what we're capable of when we slow down and let good ingredients do their thing.

Ingredients

• Puff pastry (1 sheet, about 250 g): Let it thaw at room temperature—rushing this step means it'll crack and break apart when you're trying to fit it into the pan, which I learned the hard way.
• Olive oil (2 tablespoons) and unsalted butter (1 tablespoon): The butter adds richness to those onions while olive oil prevents it from burning; this combination is worth not skipping.
• Yellow onions (2 large, thinly sliced): Thin slicing matters because it helps them cook evenly and caramelize into silky ribbons rather than chunks.
• Cremini or button mushrooms (400 g): Cremini have slightly more flavor than button mushrooms, but button mushrooms work beautifully and are often cheaper—slice them about the same thickness as your onion slices.
• Garlic (2 cloves, minced): Fresh garlic added at the very end preserves its bite and prevents it from burning into bitterness.
• Eggs (3 large), heavy cream (150 ml), and Gruyère cheese (100 g, grated): This custard-like filling is what transforms the tart from a simple vegetable dish into something silky and luxurious—don't skip the cream or use a lighter substitute or the texture won't be the same.
• Fresh thyme (1 tablespoon or 1 teaspoon dried): Thyme loves mushrooms and onions in ways that other herbs don't quite manage; if you only have dried, it'll still work wonderfully.
• Salt (1 teaspoon) and black pepper (1/2 teaspoon): Taste your vegetable mixture before the final bake and adjust seasoning—the pastry and cheese add their own salt too.
• Fresh chives or parsley for garnish: A handful of green on top isn't just pretty; it adds a fresh note that cuts through the richness.

Instructions

Get your oven and pastry ready:

Preheat your oven to 200°C (400°F) and line a 23 cm tart pan with parchment paper. Thaw your pastry completely, then press it gently into the pan, trim any wild edges, and prick the base all over with a fork—this prevents it from puffing up unevenly during baking.

Chill while you prep:

Pop the pastry-lined pan into the fridge while you work on everything else. This ten-minute rest prevents it from shrinking when it hits the heat.

Build your onion and mushroom base:

Heat the olive oil and butter together over medium heat until the butter stops foaming, then add your thinly sliced onions. Stir every couple of minutes and let them cook for about 12 minutes until they're soft, golden, and smell absolutely incredible—this is the caramelization that makes the whole dish sing.

Let the mushrooms join the party:

Add your sliced mushrooms to the pan and cook for another 8 to 10 minutes, stirring occasionally. You'll notice them releasing their liquid at first—that's normal, and the liquid will eventually evaporate, leaving behind mushrooms that are golden and concentrated in flavor. This is where patience pays off.

Finish the vegetable mixture:

Stir in your minced garlic and thyme, cook for just one more minute until fragrant, then remove the pan from heat and let everything cool slightly. Warm vegetables poured directly into the custard can scramble the eggs, so this brief cooling is important.

Make your creamy custard filling:

In a mixing bowl, whisk together your eggs, heavy cream, salt, and pepper until smooth and well combined. Stir in the grated cheese, which will distribute evenly through the mixture and create those pockets of richness in the finished tart.

Assemble and bake:

Spread the cooled mushroom and onion mixture evenly over the chilled pastry shell, then pour the egg and cheese custard over the top. Slide it into your preheated oven and bake for 25 to 30 minutes until the filling is set (it should have just a slight jiggle in the very center, not wave around) and the pastry edges are golden brown and puffed.

Rest and serve:

Let the tart cool in the pan for ten minutes before you slice it—this gives the custard time to fully set and makes slicing infinitely cleaner. Fresh herbs scattered over the top and a light squeeze of lemon juice brighten everything just before serving.

Save

What strikes me most about making this tart is how it transforms a handful of humble ingredients into something that feels completely refined. It's the kind of dish where you understand why people fall in love with French cooking—not because it's fussy, but because it respects the ingredients enough to let them become their best selves.

Why Caramelization Changes Everything

The secret magic in this tart lives in those caramelized onions. When onions cook slowly, their natural sugars brown and concentrate, creating flavors that are sweet and deep in ways that raw or quickly cooked onions simply cannot achieve. The first time I understood this difference—really understood it, not just knew it intellectually—was when I made two versions of this tart side by side and tasted them. The difference made me a believer in the power of patience.

Cheese Choices and What They Mean

Gruyère is traditional here because it melts smoothly into the custard and has enough nutty depth to hold its own against the earthiness of mushrooms. But that doesn't mean it's your only option. Goat cheese brings tanginess that some people find addictive, feta adds a sharper bite, and even a mild cheddar works if that's what you have on hand. I've made versions with all three, and while the tart tastes different each time, it's never less than delicious—which is the best kind of permission to cook with what makes sense in your kitchen.

• Avoid very soft cheeses like fresh mozzarella, which won't melt evenly and can create watery pockets.
• Grate your cheese fresh rather than using pre-grated if you have the choice—it melts more smoothly into the custard.
• Taste the filling before baking and adjust seasoning, keeping in mind that the pastry and cheese both contribute salt.

Beyond Just Dinner

This tart lives in that beautiful space between a main dish and an appetizer. Serve a smaller slice as a starter before a light soup, or make it the centerpiece of a simple lunch with a crisp green salad and a glass of cool white wine. It's equally at home at a picnic in its cooled form or fresh from the oven at a weeknight dinner table. Once you've made it, you'll find yourself reaching for it in situations where you want something that feels special but doesn't demand hours of your attention.

Save

This tart has become one of those recipes I return to again and again because it's the kind of food that makes people slow down and pay attention. There's something worth holding onto in that.

Questions & Answers

→ What type of mushrooms work best?

Cremini or button mushrooms provide a mild, earthy flavor that complements the caramelized onions nicely.

→ Can I make the tart in advance?

Yes, you can prepare the tart ahead and reheat it gently before serving to retain its flaky texture.

→ What cheese is ideal for this tart?

Gruyère or Swiss cheese adds creamy richness, but goat cheese or feta offer a pleasant twist.

→ How do I achieve caramelized onions?

Cook sliced onions slowly in olive oil and butter over medium heat until soft and golden, about 12 minutes.

→ Is this dish suitable for vegetarians?

Yes, this tart contains no meat and incorporates eggs, dairy, and vegetables for a balanced vegetarian option.