Heat oil in a skillet over medium heat. Sauté diced onion and pepper until the onions are translucent and everything is softened.
Set aside 1 cup of marinara sauce and 1 cup of shredded mozzarella cheese.
Put everything else (including the remaining marinara and cheese) in a large glass mixing bowl. Use your hands to mix until uniform.
Form into a meatloaf in the center of a 9x13-inch pan. Top with remaining marinara sauce.
Bake for 50 minutes. Sprinkle with remaining cheese and bake 10 more minutes or until the cheese is melted and the center of the meatloaf registers at least 160°F on a meat thermometer.