1. **Prep:** Preheat oven to 375°F (190°C). Lightly grease a 9×13 inch baking dish. Cook pasta shells in generously salted water for 1-2 minutes less than package directions for al dente. Drain, rinse with cool water to stop cooking, and set aside.

2. **Make Filling:** In a large bowl, combine the finely chopped shrimp, crab meat, ricotta cheese, 1 cup of the mozzarella cheese, Parmesan cheese, 3 minced garlic cloves, fresh parsley, and the egg. Season with 1 tsp salt and 1/2 tsp pepper. Mix gently until just combined.

3. **Stuff Shells:** Generously fill each cooked pasta shell with the seafood mixture. Arrange the filled shells snugly, open-side up, in the prepared baking dish.

4. **Make Sauce:** In a medium saucepan, melt the butter over medium heat. Add the 2 remaining minced garlic cloves and sauté for 30-60 seconds until fragrant. Whisk in the flour and cook for 1 minute, stirring constantly, to form a roux.

5. **Thicken Sauce:** Gradually whisk in the milk until the mixture is smooth. Bring to a simmer and continue to cook for 3-5 minutes, stirring frequently, until the sauce has thickened enough to coat the back of a spoon. Remove from heat and stir in 1/2 tsp salt, 1/4 tsp pepper, and the pinch of nutmeg.

6. **Assemble & Bake:** Pour the garlic cream sauce evenly over the stuffed shells in the baking dish. Top with the remaining 1 cup of mozzarella cheese.

7. **Bake:** Bake for 25-30 minutes, until the sauce is bubbly and the cheese on top is melted and golden brown. Let the dish rest for 5-10 minutes before serving.