01 - Preheat oven to 400°F. Place potatoes in a large pot, cover with cold water, and add a pinch of salt. Bring to a boil and simmer for 15-18 minutes until fork-tender.
02 - While potatoes cook, heat olive oil in a large skillet over medium heat. Add onion, garlic, carrots, and celery. Sauté for 5 minutes until softened.
03 - Add ground beef to the skillet. Cook, breaking it up with a spoon, until browned and no longer pink, about 6-8 minutes. Drain excess fat if needed.
04 - Stir in tomato paste, Worcestershire sauce, thyme, rosemary, salt, and pepper. Cook for 2 minutes until fragrant.
05 - Add beef broth and bring to a simmer. Stir in frozen peas and cook for 5 minutes, allowing the mixture to thicken slightly. Remove from heat.
06 - Drain the cooked potatoes. Mash with milk, butter, salt, and white pepper until smooth and creamy.
07 - Spread the beef filling evenly in a 2-quart baking dish. Top with mashed potatoes, spreading to cover completely. Use a fork to create ridges for extra crispiness.
08 - Bake for 25 minutes or until the top is golden brown and the filling is bubbling at the edges. Let stand for 10 minutes before serving.