Place the ground beef, Italian sausage, breadcrumbs, egg, onion, garlic, Italian seasoning, salt, and black pepper in a large bowl. Add 1/4 cup of the marinara sauce.
Mix the ingredients with your hands until evenly combined, but avoid overworking the meat.
Shape the mixture into a loaf and place it in a loaf pan or on a parchment-lined baking sheet.
Spread the remaining 3/4 cup marinara sauce evenly over the top of the loaf.
If freezing, cover tightly with plastic wrap and foil, then store in the freezer for up to three months.
When ready to cook, thaw overnight in the refrigerator if frozen. Bake at 375°F for 50 to 60 minutes until the internal temperature reaches 160°F.
Let the meatloaf rest for 10 minutes before slicing. Sprinkle with Parmesan cheese and parsley just before serving.