Sauté Vegetables: Heat olive oil in a small skillet over medium-high heat. Sauté the chopped onion and bell pepper for 3-4 minutes until softened. Remove from heat.
Combine Ingredients: In a large bowl, combine the ground beef, Italian sausage, sautéed onion and peppers, Worcestershire sauce, minced garlic, oregano, basil, parsley, beaten egg, Italian bread crumbs, crumbled white bread, milk, mozzarella cheese (reserve 3/4 C for topping), parmesan cheese, and 1 cup of marinara sauce.
Mix Thoroughly: Use your hands to mix all the ingredients together until well combined.
Shape and Place in Baking Dish: Place the meat mixture in a baking dish of your choice, shaping it into a loaf.
Add Marinara Sauce: Spoon 1/2 to 3/4 cup of marinara sauce over the meatloaf, spreading it evenly.