Place the chicken breasts between two sheets of plastic wrap and pound to 1/4-inch thickness.
Dip the chicken in egg, then coat with bread crumbs. Place on a foil-covered baking sheet and bake in a preheated 400-degree oven for 10 minutes.
Top the chicken with mozzarella cheese and continue baking an additional 5 minutes or until chicken is done and cheese is melted.
Meanwhile, heat the oil in a Dutch oven or deep skillet on medium heat. Add garlic; cook and stir for 1 minute. Add half the spinach; cook 1 minute or just until spinach starts to wilt, turning constantly with tongs. Add remaining spinach; cook 1 minute or just until it wilts, turning occasionally.