In a medium sauce pan, heat 2 tablespoons of the olive oil over medium low heat. Stir in the peppers, carrot, celery, onion, and garlic. Add a pinch of salt and a pinch of black pepper. Cook and stir until the vegetables are tender, about 15 minutes.
Add the crushed tomatoes, diced tomatoes, Italian seasoning, butter, crushed pepper flakes, salt, and pepper. Bring the sauce to a low boil, stirring frequently. Reduce heat to low and simmer uncovered for 30 minutes, stirring occasionally.
Preheat oven to 350 degrees.
Add the remaining oil to a large oven proof skillet. Brown the meatballs in batches in the oil over medium heat, rotating to brown all sides. Turn stove top off.
When all the meatballs are browned, arrange them all in the bottom of the skillet. Pour the prepared marinara sauce over the top.
Tear the mozzarella into chunks and arrange on top of the sauce-covered meatballs. Sprinkle with Parmesan or Asiago cheese. Bake uncovered for 30 minutes until the cheese is melted and golden.
Serve hot with additional Parmesan cheese and crusty bread.