Boil the shells

• Cook the jumbo shells in salted boiling water until al dente.
• Drain, toss lightly with oil so they don't stick, and set aside.

Make the meat mixture

• In a large bowl, combine the ground beef, Italian sausage, parsley, spinach, Parmesan, breadcrumbs, salt, pepper, garlic and onion.
• Mix until fully combined.

Fill the shells

• Scoop about a quarter cup of meat mixture into each shell.
• Leave the shells open and flat like a half circle, filled across the top.

Assemble

• Spread the spicy marinara over the bottom of a large skillet or baking dish.
• Arrange the shells face-up in a circular pattern.
• Pour the remaining marinara over the top, then sprinkle with mozzarella and Parmesan.

Bake covered

• Cover with foil and bake at 190 °C (375 °F).

Broil and finish

• Remove the foil and broil until the cheese is melted, bubbly and slightly crispy on top.