Preheat your oven to 375°F (190°C). Cook jumbo pasta shells in salted boiling water until al dente, about 9-11 minutes. Drain and set aside.

In a skillet, heat olive oil and sauté onion and garlic for 3-4 minutes until translucent.

Add fresh spinach and wilt for about 2 minutes, then remove from heat to cool slightly.

In a mixing bowl, combine ricotta, cream cheese, shrimp, crab meat, half the mozzarella, salt, pepper, and optional red pepper flakes. Fold in the cooled spinach mixture.

Spread half the marinara sauce in a baking dish and stuff each shell with the filling before arranging them beside each other.

Pour remaining marinara sauce over the shells and top with the rest of the mozzarella and Parmesan cheese.

Cover with foil and bake for 25 minutes, then uncover and bake for an additional 10 minutes until cheese is bubbly and golden.

Let cool slightly before serving to allow everything to set beautifully.