1Boil the shells — Cook jumbo shells in salted water for about 8 minutes until al dente. Drain, toss lightly with oil so they do not stick, and set aside.
2Mix the filling — In a large bowl, combine ground beef, sausage, parsley, spinach, Parmesan, breadcrumbs, salt, pepper, garlic, and onion. Mix until fully combined.
3Fill the shells — Scoop about 3-4 Tbsp (about 1/4 cup) of meat mixture into each shell. Do not close them; leave them open and flat like a half circle, filled across the top. Optional, stuff each with a cube of mozzarella.
4Assemble the dish — Spread 24 oz marinara on the bottom of a large skillet or baking dish. Arrange shells face-up in a circular pattern. Pour 30-40 oz more marinara over the top. Sprinkle with mozzarella and Parmesan.
5Bake the shells — Cover with foil and bake at 375°F for 40-45 minutes.
6Broil the topping — Remove foil and broil for 2-5 minutes until cheese is melted, bubbly, and slightly crispy on top.