Prepare the filling: In a bowl, mix the mozzarella cheese, spinach, mushrooms, garlic, salt and pepper.
Stuff the chicken: Open each chicken breast like a book and place a portion of the filling in the center. Roll each breast and secure with toothpicks.
Bread: Dip each chicken roll in the beaten egg and then in the breadcrumbs.
Cooking: Heat a little olive oil in a large skillet over medium heat. Cook the chicken rolls on all sides until golden brown and the chicken is cooked.
Serve: Serve the chicken rolls with a side dish of your choice, such as rice, potatoes or steamed vegetables.
Tips:

Vegetables: You can vary the filling by using other vegetables such as peppers, zucchini or carrots.
Cheese: Use the cheese you like the most, such as feta cheese, cheddar cheese or blue cheese.
Flavor: Add fresh herbs such as parsley, basil or thyme for an extra touch of flavor.
Topping: Instead of breading, you can top the rolls with tomato sauce and mozzarella cheese and bake them until the cheese is golden and bubbly.