01 - Preheat oven to 350°F. Lightly grease a 9 x 13-inch baking dish with butter or cooking spray.
02 - Boil jumbo pasta shells in salted water until al dente according to package directions. Drain and rinse under cold water to halt cooking process. Set aside.
03 - Combine shrimp, crab, ricotta, cream cheese, Parmesan, egg, parsley, chives, garlic, salt, pepper, and red pepper flakes in a large bowl. Mix thoroughly until fully incorporated. Set aside.
04 - Melt butter in a saucepan over medium heat. Whisk in flour and cook for 1 minute to form a roux. Gradually whisk in milk and heavy cream until smooth, ensuring no lumps remain.
05 - Bring mixture to a gentle simmer, stirring frequently for 3-4 minutes until thickened. Stir in Parmesan, lemon zest, lemon juice, nutmeg, salt, and pepper. Remove from heat.
06 - Spread one-third of sauce over the bottom of the prepared baking dish. Fill each pasta shell with approximately 2 tablespoons of seafood mixture. Arrange stuffed shells open side up in the dish.
07 - Pour remaining white sauce evenly over the shells. Sprinkle mozzarella and additional Parmesan cheese across the top.
08 - Cover with aluminum foil and bake for 25 minutes. Remove foil and continue baking for 10 additional minutes until cheese is bubbly and golden brown.
09 - Allow dish to rest for 5 minutes before serving. Garnish with additional fresh parsley if desired.