Begin by cooking the jumbo pasta shells according to the package instructions until they are al dente, about 8-10 minutes. Drain and rinse under cold water.

In a medium skillet over medium heat, melt 1 tablespoon of butter. Add the chopped celery, green pepper, and red onion, sautéing for about 5 minutes.

In a large mixing bowl, combine the lump crabmeat, cooked shrimp, mozzarella cheese, and cooled sautéed vegetables. Add seasoning, ground pepper, egg, mayonnaise, and milk.

Preheat your oven to 350°F (175°C). Stuff each cooled shell with a rounded tablespoon of the seafood filling and arrange them in a greased baking dish.

In a saucepan, melt the remaining butter. Whisk in the flour and cook for 1-2 minutes. Gradually add milk while whisking until thickened, then stir in Parmesan.

Pour the sauce over the stuffed shells and sprinkle with additional seasoning. Bake uncovered for 30-35 minutes until bubbly and golden.