Flatten chicken breasts by pounding them and then season with salt and pepper. Whisk egg and milk together in a bowl and put bread crumbs in another bowl and season with salt and pepper, then add garlic and pepper flakes. Dip the chicken first in the egg mixture, then in the bread crumbs, coating thoroughly.
Heat a generous amount of Avocado Oil in a skillet and brown the chicken pieces, a few at a time for around 4 minutes per side or until golden. Grease an oven dish with avocado oil and arrange chicken pieces inside. Choose a dish that will make a snug fit for the chicken.
Pour the Spaghetti sauce over the chicken, covering well. Use enough sauce to cover the chicken well. Wrap tin foil over the top of the dish and bake for about 35 minutes. Remove the foil and put the mozzarella and parmesan cheeses on top of the chicken and return, uncovered to the oven for about 5 minutes.