Start with the marinara. Heat the olive oil in a large sauce pot over medium heat. Add the diced onion and cook until soft, about 2-3 minutes. Add garlic and cook for 30 seconds.
Add tomato sauce, crushed tomatoes, sugar, crushed red pepper, Italian seasoning and 1 teaspoon EACH of salt and pepper. Bring to a boil and then lower the heat to low. Cover and let it simmer (stir occasionally) while you prepare the meatballs.
Combine all meatball ingredients in a large bowl. Mix them well using your hands. Roll the balls into golf ball-sized meatballs.
Combine the breading ingredients in a large, shallow bowl. Roll the meatballs into the breading and set on a plate or tray.Preheat your oven to 375 degrees. To a large skillet, add 1/2 tbsp of olive oil and 1/2 tbsp unsalted butter over medium-high heat. Cook the meatballs in 2 batches. Add half of the meatballs to the skillet and cook, browning on all sides. The meatballs do not need to be cooked all of the way through, they will finish cooking in the oven with the marinara.
Grab a large oven-proof skillet or baking dish and ladle some marinara into the bottom. When the first batch meatballs are done cooking, place them into the skillet with the marinara. To cook the second batch of meatballs, add the remaining half tbsp of olive oil and 1/2 tbsp butter. Cook until browned and then add to the skillet with the marinara.
Cook for 15 minutes. Remove from oven, top the meatballs with 1-2 cups mozzarella cheese and bake for another 5 minutes to melt the cheese.
Top the meatballs with fresh parsley or basil. Serve the meatballs with pasta and the remaining marinara. Enjoy!