Preheat oven to 400℉. Spray or lightly oil an 8x8 inch pan.
Place half of the sliced mushrooms evenly in the bottom of the pan. Reserve the remaining mushrooms.
Lay out three small plates. Add flour to one plate. Add beaten egg to the second plate. Add bread crumbs to a third plate.
Dip and roll the chicken in flour until the chicken is lightly covered. Roll the chicken in the beaten egg. Finally, roll the chicken in bread crumbs.
In a large skillet, melt butter over medium heat and drizzle in olive oil. Brown both sides of chicken in skillet. Place chicken on top of mushrooms, arrange remaining mushrooms on chicken, and top with mozzarella cheese. Add chicken stock or wine to pan around the sides of chicken. Season with black pepper to taste.
Bake for 30 minutes. Let rest for a few minutes and serve.