Preheat your skillet over medium-high for 2 minutes so the chicken sears instead of steams. Use a mix of butter and oil to get rich flavor without burning.

Pat chicken breasts dry with paper towels, then season both sides with salt and pepper.

Heat 1 tablespoon butter and 1 tablespoon oil in a large skillet over medium-high. Add chicken and sear for 4–5 minutes per side, until golden and cooked to 165°F internal temperature. Transfer to a plate and tent with foil.

Add sliced mushrooms to the same skillet in a single layer. Cook for 3 minutes without stirring, until browned on one side. Stir and cook 2 minutes more, then transfer to the plate with chicken.

Lower heat to medium. Add minced garlic and diced onion to the pan. Sauté for 1 minute, until the garlic smells sweet and the onion softens. Pour in ½ cup chicken broth or white wine, scraping up browned bits from the pan bottom.

Pour in 1 cup heavy cream and stir gently. Simmer for 3 minutes, until the sauce thickens enough to coat the back of a spoon. Return chicken and mushrooms to the skillet, spoon sauce over the top, and simmer for 2 minutes to reheat. Sprinkle with fresh thyme leaves and serve.