Finely dice the onions. Mince the garlic.
For the cheese, take 2 slices at a time and then quarter them and stack the quarter slices. You will have 6 stacks of 8 slices that will be used to fill the meatballs/patties.
In a large mixing bowl, add the ground beef, onion, garlic, 1 cup breadcrumbs, parmesan cheese, eggs, salt, and pepper. Using your hands, mix to completely combine all of the ingredients.
Divide the mixture in half in the bowl and then divide each half into thirds, so that you can make sure to get 3 patties from each half, for a total of 6. Take one of the 6 sections of beef and form it into a 1-in thick patty. Place one of the 6 stacks of mozzarella cheese in the center of the patty and them using your fingers, gently fold the sides of the patty up around the cheese so you can no longer see any of the cheese. Place the meatball on a baking sheet and gently press to flatten it slightly. Then do the same for the other 5 patties.
In a medium bowl, add the 2 cups of breadcrumbs, oregano, 1 teaspoon of salt, and 1 teaspoon of pepper. Mix to combine. For each of the patties, gently press them in the breadcrumbs to coat them with a light layer of breading. The meat should be wet enough so that the breadcrumbs stick. But if not, you can beat an egg and first dip the patties into the egg, then the breadcrumbs. Place back on the sheet pan.
Spray the tops of the Polpettone with olive oil. Then gently flip them over and spray the other side.
Bake in a 400 F degree oven for 40 minutes until golden brown.
Wile the Polpettone are baking, warm up the marinara sauce. I like to serve the Polpettone on top of the sauce, so the tops stay crispy and crunchy and each bite can be dipped into the sauce. Enjoy!