Preheat oven to 375°F.
Pound chicken breasts to 1/2 inch thickness.
Season both sides with salt and pepper.
Heat oil in large skillet over medium heat.
Sauté mushrooms until golden brown, about 5 minutes.
Add garlic and cook for 1 minute.
Add spinach and cook until wilted.
Stir in cream cheese until melted.
Add heavy cream and mozzarella cheese.
Mix until creamy and smooth.
Cut pocket in each chicken breast.
Stuff each breast with mushroom mixture.
Secure with toothpicks to seal edges.
Sear chicken in skillet for 3 minutes.
Flip and sear other side.
Transfer to oven-safe dish.
Bake for 20-25 minutes until cooked through.
Rest for 5 minutes before serving.