Preheat oven to 375°F (190°C) and lightly grease a 9x13 inch baking dish.

Cook jumbo shells in a large pot of boiling salted water until al dente according to package instructions; drain and rinse with cold water to prevent sticking.

In a medium skillet, melt 1 tablespoon of butter over medium heat and sauté the chopped shrimp until pink and opaque, approximately 3 minutes; remove from heat and allow to cool.

In a large mixing bowl, combine the ricotta cheese, 0.75 cups mozzarella, 0.5 cups parmesan, the beaten egg, sautéed shrimp, crab meat, minced garlic, and parsley.

In a medium saucepan, melt the remaining butter over medium-low heat. Whisk in the heavy cream and bring to a gentle simmer.

Gradually whisk in the remaining 0.5 cups of parmesan cheese until the sauce is smooth and thickened; season with salt, pepper, and nutmeg.

Spread 0.5 cups of the prepared cream sauce across the bottom of the baking dish.

Generously fill each cooked pasta shell with the seafood and cheese mixture and arrange them in the baking dish in a single layer.

Pour the remaining cream sauce evenly over the stuffed shells and top with the remaining 0.75 cups of mozzarella cheese.

Bake for 25 to 30 minutes until the cheese is melted and the sauce is bubbling.

Let the dish rest for 5 minutes before serving to allow the internal temperature to stabilize.