Spray a 9″x 5″ loaf pan with nonstick cooking spray.
Grab a large bowl, mix the ground beef, 1 cup of the marinara sauce, diced red onion, eggs, Parmesan cheese, Italian seasoning, and salt and pepper. Use your hands. Getting messy is swell!
Fill loaf pan with the meatloaf mixture, patting with your hands to form an even top.
Bake uncovered for 45 minutes.
Remove loaf from oven and then top with the remaining 1⁄2 cup of marinara sauce, and all of the mozzarella cheese. Return to oven and bake an additional 30 minutes (Your meat thermometer (you have one, right?) should register 165 degrees.
Drain fat from pan.
Let the loaf rest 10 minutes
Sprinkle with oregano (or use a decorative sprig bit) and cut 8 equal slices.