Cook the pasta from the seafood stuffed shells recipe in a large pot, according to packaging instructions. Typically, you will warm up enough water in a pot to cover the pasta and then add the pasta when the water is boiling. From there, cook the pasta until tender.
Beat the egg and set aside.
Sauté the onion in butter using a skillet and then set that aside.
Combine egg, shrimp, mozzarella cheese, crab, 2 tablespoons of milk, mayonnaise, pepper, and seafood seasoning.
Preheat your oven to 350 degrees Fahrenheit and prepare a 13 x 9 baking dish by greasing it.
Drain the pasta and rinse it, then stuff each shell with about a tablespoon of seafood mixture.
Place the shells into the baking dish, but don’t bake yet.
Melt the rest of your butter in a separate saucepan on medium heat and add in the flour and pepper. Slowly mix in the rest of the milk and bring the mixture of a boil. Cook for an additional 2 minutes and then stir in the Parmesan cheese. You have just made cheesy white sauce, also called a roux.
Pour the roux over the stuffed shells and garnish with seafood seasoning. Bake for 30 minutes. You will know the seafood stuffed shells are done cooking when they are bubbly.