In a large non-stick pan or skillet, heat 1 tablespoon of olive oil over medium heat. Add minced garlic and sliced mushrooms. Sauté until mushrooms are tender, season with salt and pepper, then set aside in a bowl with juices.
Cut a pocket into each chicken breast. Season with salt, pepper, onion powder, and Italian seasoning. Stuff each pocket with 2 slices of mozzarella cheese and sautéed mushrooms, then seal with a toothpick.
Heat the remaining oil in the pan and sear the chicken until golden on both sides, about 6-8 minutes. Set aside.
In the same skillet, cook minced garlic for about a minute. Add Dijon mustard and gluten-free flour, whisking. Slowly pour in almond milk, whisking to form a thick sauce. Stir in Parmesan cheese, salt, and pepper. Add any remaining mushrooms and juices.
Return the chicken to the pan, spoon sauce over it, cover, and cook over low heat for 12-15 minutes until chicken reaches 165°F internally.
Garnish with fresh parsley and serve warm.
Notes
For a crispy topping, broil the chicken for a few minutes after cooking.