Step 1: Prepare the Mushroom Filling
Heat 1 tbsp olive oil and 1 tbsp butter in a large skillet over medium heat.
Add the sliced mushrooms and cook for about 5 minutes, until softened.
Stir in the garlic, cooking until fragrant (about 1 minute).
Remove the skillet from heat, add mozzarella and Parmesan cheese to the mushrooms, and stir to combine. Set aside.
Step 2: Prepare the Chicken Breasts
Preheat your oven to 375°F (190°C).
Using a sharp knife, slice each chicken breast horizontally to create a pocket, being careful not to cut all the way through.
Stuff each chicken breast with the mushroom and cheese filling, using toothpicks to secure the open sides if necessary.
Step 3: Sear the Chicken
Heat the remaining 1 tbsp olive oil and 1 tbsp butter in the skillet over medium-high heat.
Season the stuffed chicken breasts with salt, pepper, and Italian seasoning, then carefully place them in the hot skillet.
Sear the chicken for about 3-4 minutes on each side until golden brown.
Step 4: Bake the Chicken
Transfer the seared chicken breasts to a baking dish and bake in the preheated oven for 15-20 minutes, or until the chicken is cooked through and the cheese is melted.
Remove from the oven and let rest for a few minutes before serving.
Step 5: Serve and Garnish
Garnish with fresh chopped parsley if desired, and serve hot.