Preheat your oven to 375°F (190°C).
In a large mixing bowl, combine the ground beef, ground pork, breadcrumbs, eggs, Worcestershire sauce, garlic powder, onion powder, salt, and pepper. Mix until all ingredients are well incorporated.
Fold in the diced bell peppers, chopped onions, and frozen mixed vegetables, distributing them evenly throughout the meat mixture.
In a greased loaf pan or casserole dish, spread half of the meat mixture evenly along the bottom.
Layer half of the marinara sauce over the meat mixture and sprinkle half of the shredded cheese over the sauce.
Add the remaining meat mixture on top and finish with the rest of the marinara sauce.
Top with any remaining cheese and sprinkle with additional cheese if desired.
Cover the casserole with aluminum foil and bake in the preheated oven for 30 minutes.
After 30 minutes, remove the foil and bake for an additional 15 minutes, or until the cheese is bubbly and golden brown.
Allow the casserole to cool for a few minutes before slicing and serving.