Preheat oven to 350°F (175°C) and grease a 9x5-inch loaf pan or line a baking sheet with parchment paper.
In a small skillet, sauté diced onion and minced garlic over medium heat until translucent, approximately 5 minutes. Set aside to cool.
In a large mixing bowl, combine the ground beef, Italian sausage, breadcrumbs, beaten eggs, milk, Parmesan cheese, parsley, oregano, salt, pepper, and 1/2 cup of the marinara sauce.
Add the cooled onion and garlic mixture to the bowl.
Mix by hand until just combined, being careful not to overwork the meat to ensure a tender texture.
Transfer the mixture to the prepared loaf pan or shape into a loaf on the baking sheet.
Bake for 45 minutes.
Remove from oven and spread the remaining 1/2 cup of marinara sauce evenly over the top.
Return to the oven and bake for an additional 20-25 minutes, or until the internal temperature reaches 160°F (71°C).
Allow the meatloaf to rest for 10 minutes before slicing to retain juices.