01 - Place chicken breasts between plastic wrap sheets and pound gently to achieve even 1/2-inch thickness.
02 - Combine flour, salt, and black pepper in a shallow bowl. Coat each chicken breast thoroughly in the seasoned flour mixture, shaking off excess.
03 - Heat olive oil in a large skillet over medium-high heat. Add chicken breasts and cook 4-5 minutes per side until golden brown and cooked through. Transfer to a plate and tent loosely with foil.
04 - In the same skillet, melt butter over medium heat. Add sliced mushrooms and cook, stirring occasionally, for 5-6 minutes until browned and tender.
05 - Add minced garlic to the mushroom mixture and cook for 1 minute until fragrant.
06 - Pour in chicken broth, grape juice, balsamic vinegar, and thyme. Bring to a simmer, scraping any browned bits from the pan bottom. Simmer for 5-7 minutes until sauce reduces slightly.
07 - For a thicker sauce consistency, stir in the cornstarch slurry and cook for 1-2 minutes until thickened.
08 - Return seared chicken to the skillet, spooning sauce and mushrooms over the top. Simmer for 2-3 minutes to heat through completely.
09 - Transfer chicken and sauce to serving plates. Sprinkle with fresh parsley before serving.