Heat olive oil in a medium saucepan over medium heat. Add onion and cook until soft and translucent, about 5 minutes. Stir in garlic and cook 30 seconds until fragrant.

Add crushed tomatoes, basil, remaining 1/2 teaspoon salt, and red pepper flakes if using. Bring to a gentle simmer, reduce heat to low, and cook uncovered for 20 minutes, stirring occasionally, until slightly thickened.

In a large bowl, combine panko and milk. Let sit 2 minutes so the crumbs absorb the liquid.

Add beef, pork, egg, Parmesan, garlic, oregano, 1 teaspoon salt, and pepper to the soaked breadcrumbs. Use your hands to mix gently until just combined - do not overwork or the meatloaf will be tough.

Transfer mixture to a 9x5-inch loaf pan or shape into a free-form loaf on a foil-lined rimmed baking sheet. Press gently to form an even loaf about 2 inches high.

Preheat oven to 350F. Spread 1 cup of marinara over and around the meatloaf, reserving the rest for serving. Bake 45 minutes.

Remove from oven, arrange mozzarella over the top, and return to oven for 15-20 minutes until cheese is melted and bubbly and an instant-read thermometer inserted in the center reads 160F. Let rest 10 minutes before slicing. Serve with warm reserved marinara on the side.