Preheat oven to 375°F and line a baking sheet with parchment paper or lightly grease a loaf pan.

In a large mixing bowl, combine ground beef, Italian sausage, chopped onion, minced garlic, bread crumbs, eggs, Parmesan cheese, Italian seasoning, and parsley. Pour in milk and season with salt and pepper. Mix gently with hands until just combined.

On prepared baking sheet, pat meat mixture into a flat rectangular shape about 9x12 inches and roughly 3/4 inch thick. Use damp hands to prevent sticking.

Create a shallow channel down the center of the rectangle, leaving about 2 inches of space on all sides. Layer shredded mozzarella, chopped spinach, and 1/4 cup marinara sauce into this channel.

Carefully lift the long edges of the meat and fold them over the filling, pinching the seams together firmly until you feel them bond with no gaps. Gently roll the loaf seam-side down and shape into a smooth, even log.

Spread remaining 1/2 cup marinara sauce over the top and sprinkle with 1/4 cup Parmesan cheese.

Bake for 45-55 minutes until internal temperature reaches 160°F on a meat thermometer inserted into the thickest part. The top should be golden brown with slightly darker, caramelized edges.

Let the meatloaf rest for 10 minutes before slicing to allow juices to redistribute and ensure cleaner slices.