Cook the large pasta shells in boiling salted water according to package instructions, usually 9–11 minutes until al dente. Once done, drain and rinse under cool water.

In a large mixing bowl, combine chopped seafood with softened cream cheese and seasonings until smooth.

Melt 4 tablespoons of butter in a medium saucepan over medium heat. Add minced garlic and chopped onion and sauté for about 2-3 minutes. Gradually whisk in flour, cooking for another minute, then add milk and stir until thickened. Incorporate cheddar cheese until melted.

Preheat oven to 180°C (160°C Fan). Fill each shell with the seafood mixture, arrange in a greased baking dish, and pour the cheese sauce over them.

Cover with foil and bake for 25-30 minutes. Remove foil for the last 5 minutes to brown the top.