Preheat oven to 400° F. Very lightly spray a regular-sized muffin pan with non-stick cooking spray. Heat a medium skillet over medium to medium-high heat. Add oil. Once oil is hot, stir in onion. Sauté for 3 to 5 minutes, until onion is softened, stirring occasionally. Then add garlic and sauté for 3 minutes more. Remove pan from heat. Transfer onion and garlic mixture to a large bowl. Let cool for a few minutes. To the onion and garlic mixture, stir in 1/2 cup of the marinara, and then the egg, bread crumbs, Parmesan, parsley, mustard, oregano, salt, and pepper. Stir to combine. Add ground beef and Italian sausage, and combine thoroughly with your hands. Lightly press about half of the meatloaf mixture into each of 12 muffin cups. Press a cube of mozzarella gently into the center of each muffin cup. Top each cube of cheese with more meatloaf mixture, evenly dividing up the remaining mixture. Top meatloaf muffins with remaining 1/2 cup of marinara, dividing evenly amongst the 12 portions. Place muffin pan on a rimmed baking sheet and bake for about 30 minutes, or until internal temperature registers 165° F and the muffins are hot and bubbling. Remove pan from oven and let stand for a few minutes. Using a slotted spoon, carefully transfer meatloaf muffins to a serving platter. Sprinkle with additional chopped parsley, if desired.