In a large skillet, melt 1 tablespoon of butter over medium-high heat. Add the mushrooms and sauté for 5-6 minutes until golden brown. Stir in half of the minced garlic and 1 tablespoon of parsley; cook for 1 minute until fragrant. Season with salt and pepper, then remove from heat and set aside.

Pat the chicken breasts dry. Using a sharp knife, cut a horizontal pocket into the side of each breast, being careful not to cut all the way through.

Season the inside of the pockets with salt and pepper. Stuff each breast with a slice of Mozzarella cheese and a generous spoonful of the mushroom mixture. Secure the edges with 2-3 toothpicks.

Rub the outside of the chicken with olive oil, oregano, onion powder, salt, and pepper.

In the same skillet used for the mushrooms, heat the remaining tablespoon of butter over medium-high heat. Sear the chicken for 3-5 minutes per side until beautifully golden.

Reduce heat to medium, add the remaining garlic to the pan, and cover with a lid. Cook for another 10-15 minutes (or transfer to an oven at 375°F) until the internal temperature reaches 165°F.

Remove toothpicks carefully. Spoon any remaining garlic butter from the pan over the chicken. Garnish with the rest of the fresh parsley and serve immediately.