Preheat the oven to 400 degrees F (200 degrees C).
Toast the Bread (1 slice) and pulse in the food processor to make crumbs.
Cover the bread crumbs in Milk (1/4 cup) and allow to stand until all the milk is absorbed.
Mince the Onion (1) and put in a small bowl.
Grate the Mozzarella Cheese (3/4 cup) and Parmesan Cheese (1/4 cup), and set aside.
Combine Ground Chicken (1 pound), bread crumbs, minced small onion, Sour Cream (1 tablespoon), Salt (1 teaspoon) and Ground Black Pepper (1/2 teaspoon) in a bowl. Mix thoroughly.
Shape the mixture into meatballs, about 1 tablespoon big each.
Place the meatballs on a baking sheet lined with aluminum foil and sprayed with oil.
Apply the Mayonnaise (1/4 cup) evenly on the top of each meatball.
Combine the two cheeses together and sprinkle about 1/2 tablespoon on each meatball.
Bake in the oven uncovered for 15-20 minutes.
Broil for another 2-5 minutes to brown and crisp up the cheese.
Serve and enjoy warm.