Preheat your oven to 375°F.

Cook the jumbo pasta shells according to package instructions until al dente. Drain and set aside.

In a large skillet, heat the olive oil over medium heat. Add the chopped onion and garlic, sautéing until the onion is translucent, about 3-4 minutes.

Add the shrimp and scallops to the skillet, cooking until they are just opaque, about 3-5 minutes. Remove from heat and let cool slightly.

In a mixing bowl, combine the ricotta cheese, half of the mozzarella cheese, Parmesan cheese, parsley, lemon juice, Italian seasoning, salt, and pepper. Stir in the seafood mixture until well combined.

Spread 1 cup of marinara sauce on the bottom of a 9x13 inch baking dish. Stuff each cooked pasta shell with the seafood mixture and place them in the dish.

Pour the remaining marinara sauce over the stuffed shells, then drizzle with heavy cream. Top with the remaining mozzarella cheese.

Cover the dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and golden.

Let the dish cool for a few minutes before serving.