Combine beef, sausage, onion, garlic, bread crumbs, eggs, Parmesan, Italian seasoning, parsley, milk, salt, and pepper in large bowl. Mix gently until just combined.
On parchment-lined baking sheet, shape meat into 9×12-inch rectangle about 1/2 inch thick.
Create 3-inch wide channel down center, leaving 1-2 inches from edges.
Stuff with mozzarella, spinach if using, and spoonfuls of marinara sauce.
Fold meat over filling from both sides, pinch seam firmly to seal. Flip seam-side down and shape evenly.
Spread remaining marinara over top and sprinkle with Parmesan.
Bake 45-55 minutes until internal temperature reaches 160°F in meat portion.
Let rest 10 minutes before slicing.
Slice carefully and serve with extra marinara if desired.
Notes
Italian sausage adds authentic flavor. Seal edges tightly to prevent cheese leaking.
Use meat thermometer to check doneness, inserting into meat not cheese. 160°F is safe temperature.
Can assemble day ahead and refrigerate. Can freeze uncooked for up to 3 months.