Preheat oven to 350 degrees and ready a nonstick
9x5 loaf pan for baking. or spray a regular loafpan with nonstick cooking spray. ( I use a nonstick pan because I dont like to use cooking spray).
In a large bowl, mix the ground beef, 1 cup of marinara sauce, diced red onion, eggs, parmesan cheese, italian seasoning, and salt and pepper.(this is easiest to mix with your hands.)
Evenly fill loaf pan with meatloaf mixture, patting with your hands to form an even top. Bake uncovered for 45 minutes.
Remove loaf pan from oven, drain excess fat or liquid, and then top with remaining 1/2 cup of marinara sauce, and all the mozzarella cheese. return to oven and bake an additional 30 minutes, or until meat thermometer reads 165 degrees. drain fat from pan once more and let rest for 10 minutes before cutting into 8 equal slices.