Preheat your oven to 375°F (190°C).

In a large mixing bowl, combine the ground beef, ground Italian sausage, breadcrumbs, eggs, garlic powder, onion powder, Italian seasoning, salt, and pepper.

In a separate bowl, mix together the mozzarella cheese, Parmesan cheese, and spinach (if you're using it).

Take the meat mixture and shape it into a rectangular slab about 1 inch thick. Spread the cheese filling evenly across, leaving a small border around the edges, then roll the meat over the cheese and seal the seams.

Place the rolled meatloaf seam-side down in a prepared baking dish. Bake for 45 to 50 minutes or until the internal temperature reaches 160°F (71°C).

During the last 10 minutes of baking, spread marinara sauce on top.

Once baked, remove from oven and let rest for about 10 minutes before slicing.

Slice the meatloaf and serve with your favorite sides.