Melt 4 tablespoons of unsalted butter in a large skillet over medium heat. Add the finely chopped onion and sauté for about 3-4 minutes until translucent.
Add minced garlic and cook for another minute. Sprinkle 1/4 cup of flour over the mixture, stirring continuously for about 2 minutes.
Gradually pour in 2 cups of milk, whisking constantly until thickened (about 5-7 minutes).
Remove from heat and stir in 2 cups of shredded cheddar cheese until melted.
In a mixing bowl, combine crab meat and shrimp. Fold in the cheese sauce and season with Old Bay seasoning, salt, and pepper to taste.
Transfer the mixture to a greased 9x13-inch baking dish. Combine breadcrumbs with 1 tablespoon of melted butter and sprinkle over the seafood mixture.
Bake for 25-30 minutes until golden brown on top.
Let rest for 10 minutes before serving. Garnish with chopped parsley.
Notes
This casserole can be customized with different seafood or vegetables. Serve with a fresh salad or garlic bread.