Preheat your oven to 375°F (190°C). Lightly grease a 9x5 inch loaf pan or line a baking sheet with parchment paper if making a free-form loaf.

Heat olive oil in a small skillet over medium heat. Add the finely chopped onion and cook until softened, about 5-7 minutes. Add the minced garlic and cook for another minute until fragrant. Remove from heat and let it cool slightly.

In a large mixing bowl, combine the Italian breadcrumbs and milk. Let it sit for 5 minutes to allow the breadcrumbs to absorb the milk. This creates a 'panade', which is key to a moist meatloaf.

To the bowl with the panade, add the ground beef, ground pork, cooled onion and garlic mixture, lightly beaten eggs, grated Parmesan cheese, fresh parsley, oregano, basil, salt, pepper, and optional red pepper flakes.

Using your hands, gently mix all the ingredients until just combined. Be careful not to overmix, as this can make the meatloaf tough.

Transfer the meat mixture to your prepared loaf pan and gently press it into an even layer. If using a baking sheet, shape the mixture into a free-form loaf, about 9x5 inches.

In a small bowl, whisk together the marinara sauce, balsamic vinegar, and brown sugar to create the glaze.

Bake the meatloaf for 40 minutes. Remove it from the oven and carefully spoon half of the marinara glaze evenly over the top.

Return the meatloaf to the oven and bake for another 15-20 minutes, or until the glaze is caramelized and the meatloaf is cooked through. The internal temperature should reach 160°F (71°C) on an instant-read thermometer.

Let the meatloaf rest in the pan for at least 10 minutes before slicing and serving. This allows the juices to redistribute, keeping it moist and preventing it from falling apart.