Preheat the oven to 350°F and prepare a loaf pan by lining it with parchment paper or lightly buttering it.

Heat a small skillet over medium heat and sauté the diced onion in olive oil until softened, about 3–4 minutes, then add the garlic and cook for 30 seconds more before removing from heat to cool slightly.

Pour the marinara sauce into a small saucepan and simmer over medium-low heat for 8–10 minutes, stirring occasionally, until slightly thickened and reduced, then remove from heat and let cool slightly.

In a large mixing bowl, combine the ground beef, Italian sausage, breadcrumbs, eggs, milk, Parmesan cheese, ½ cup mozzarella, parsley, Italian seasoning, salt, pepper, sautéed onion, and garlic.

Add ½ cup of the reduced marinara sauce to the meat mixture and gently mix just until combined, being careful not to overwork the meat.

Transfer the mixture into the prepared loaf pan and shape it evenly.

Spread the remaining reduced marinara sauce over the top of the meatloaf.

Bake uncovered for 45 minutes, then sprinkle the remaining ½ cup mozzarella over the top and return to the oven for 10–15 minutes, or until the internal temperature reaches 160°F.

Remove from the oven and allow the meatloaf to rest for 10 minutes before slicing and serving.