Preheat oven to 350 degrees F (175 degrees C). Lightly grease a medium baking dish.

Melt 3 tablespoons butter in a skillet, and saute the onions and celery until tender.

Heat the milk in a saucepan over medium heat, and mix in the flour and 1 1/2 teaspoons butter until well blended. Gradually blend the cheese into the mixture. Season with salt and pepper.

In a medium bowl, mix the onions and celery with the cheese sauce mixture. Toss in the crabmeat, lobster, shrimp, and scallops. Transfer to the prepared baking dish.

Bake uncovered in the preheated oven 25 minutes, or until the seafood is opaque and the surface is lightly browned.