Preheat oven to 400 degrees F. Lightly spray a 13 x 9-inch baking pan.
Mix the bread crumbs and grated cheese in a small bowl; set aside.
Make 3-4 deep cuts across the top of each chicken breast about 1-1.5-inches apart (mine were large breasts and I got 3 cuts spaced 1.5-inches apart). Set aside while making the filling
For filling--heat the oil in a large deep pan over medium heat.
Add the garlic and spinach and stir constantly until spinach is completely wilted--about 3 minutes. Remove from heat and add the mozzarella. Stir until cheese starts to melt.
Divide filling evenly among the 4 breast pieces.
Coat each chicken breast with about 1 tbs of mayonnaise.
Place in pre-heated oven and bake for 15 minutes.
Divide bread crumb mixture between the 4 chicken breasts. Return to oven and bake an additional 30-35 minutes or until chicken is done (175 degrees F on a meat thermometer or until juices run clear)
I used small cubes (1/4-inch cubes) of mozzarella that I cut myself. This melts so much better than the pre-shredded variety which has cornstarch and other additives to prevent it sticking together--I find that inhibits melting to some degree.