Step 1: Cook jumbo pasta shells according to package directions until al dente. Drain, rinse with cold water, and set aside to cool slightly to prevent sticking and tearing.
Step 2: In a large bowl, combine the chopped cooked shrimp, lump crab meat, ricotta cheese, softened cream cheese, 1/2 cup Parmesan cheese, 2 tbsp chopped parsley, garlic powder, and onion powder. Season generously with salt and pepper. Mix until well combined.
Step 3: Prepare the cream sauce: In a large saucepan or Dutch oven, melt butter over medium heat. Add minced garlic and sauté for 1 minute until fragrant. Whisk in flour and cook for 1-2 minutes, stirring constantly, to create a roux.
Step 4: Gradually whisk in the milk and heavy cream, a little at a time, until smooth. Bring the sauce to a simmer, stirring frequently, until it thickens, about 5-7 minutes. Remove from heat and stir in the Gruyere cheese and nutmeg until melted and smooth. Season with salt and pepper to taste.
Step 5: Preheat your oven to 375°F (190°C). Spread about 1 cup of the cream sauce in the bottom of a 9x13 inch baking dish.
Step 6: Carefully fill each cooked pasta shell with a generous amount of the seafood and cheese mixture. Arrange the stuffed shells in a single layer in the prepared baking dish.
Step 7: Pour the remaining cream sauce evenly over the stuffed shells. Sprinkle with additional Parmesan cheese.
Step 8: Cover the baking dish tightly with foil and bake for 20 minutes. Remove the foil and bake for another 10-15 minutes, or until the sauce is bubbly and the cheese is golden brown. If desired, you can broil for the last 2-3 minutes for a deeper golden crust (watch carefully!).
Step 9: Let the dish rest for 5-10 minutes before serving. Garnish with fresh chopped parsley.