Preheat oven to 375°F (190°C).
Prepare the filling: In a skillet over medium heat, heat olive oil. Add mushrooms and cook until they release moisture and start to brown, about 5-6 minutes. Stir in garlic, parsley, thyme, salt, and pepper. Cook another 1-2 minutes. Remove from heat and mix in Parmesan cheese.
Butterfly the chicken: Carefully slice each chicken breast horizontally to create a pocket, being careful not to cut all the way through.
Stuff the chicken: Spoon the mushroom filling into each breast, securing the edges with toothpicks or kitchen twine.
Sear: In an oven-safe skillet, melt butter over medium-high heat. Sear the chicken for 2-3 minutes per side until golden.
Bake: Transfer the skillet to the preheated oven and bake for 15–20 minutes, or until the chicken reaches an internal temperature of 165°F (74°C).
Rest and serve: Let the chicken rest for 5 minutes before slicing. Garnish with extra parsley if desired.