Preheat the oven to 375 degrees F (190 degrees C).

Whisk egg in shallow bowl. Dip and coat chicken breasts in the egg.

Heat a nonstick pan over medium-high heat. Brown chicken, flipping to ensure both sides are cooked, 5 to 7 minutes total.

Meanwhile, combine spinach, bacon, tomatoes, Cheddar cheese, mozzarella cheese, mayonnaise, and garlic in a medium bowl.

Remove chicken from the pan. Slice into the meatiest, thickest side of each breast carefully in a butterfly-cut, creating a "pocket" for the stuffing. Stuff liberally with spinach mixture.

Place stuffed chicken into a baking dish, with stuffing mixture at the top. If pieces cannot stand on their own, lean against each other or the side of the baking dish to ensure stuffing remains inside chicken. Sprinkle each with salt and pepper. Cover the dish.

Bake in the preheated oven until chicken is no longer pink in the center and juices run clear, about 40 minutes.