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 stop cooking process. Set aside on clean surface.
03 - In a large mixing bowl, combine shrimp, crab, ricotta, cream cheese, Parmesan, beaten egg, parsley, chives, minced garlic, salt, black pepper, and red pepper flakes. Mix thoroughly until fully incorporated and uniform in texture.
04 - Melt butter in saucepan over medium heat. Whisk in flour and cook for 1 minute to form roux, stirring constantly to prevent burning.
05 - Slowly whisk in whole milk and heavy cream until roux is fully dissolved and mixture is smooth. Bring to gentle simmer, stirring frequently to prevent scorching.
06 - Continue simmering sauce for 3-4 minutes until thickened enough to coat back of spoon. Stir in Parmesan, lemon zest, lemon juice, nutmeg, salt, and pepper. Remove from heat.
07 - Spread one-third of the prepared white sauce evenly across bottom of greased baking dish.
08 - Fill each cooked pasta shell with approximately 2 tablespoons of seafood filling. Arrange stuffed shells in single layer, opening facing up, in prepared dish.
09 - Pour remaining white sauce evenly over stuffed shells. Sprinkle shredded mozzarella and additional Parmesan cheese across top.
10 - Cover baking dish tightly with aluminum foil. Bake for 25 minutes until heated through and sauce is bubbling around edges.
11 - Remove foil and continue baking for 10 additional minutes until cheese is golden brown and sauce is bubbly.
12 - Let dish stand for 5 minutes to allow filling to set. Garnish with additional fresh parsley if desired. Serve hot.