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, stir together mozzarella cheese, Parmesan cheese, and spinach (if using) for the filling.

Shape the meat mixture into a large rectangle, spread the cheese filling over it, and roll it tightly.

Bake seam-side down for 45-50 minutes, or until internal temperature reaches 160°F (71°C).

Spread marinara sauce over the meatloaf in the last 10 minutes of baking.

Let the meatloaf rest for 10 minutes before slicing and serving.