Chop celery stalks, green onions, and onion.

Saute onions and celery in margarine until softened.

Blend flour into evaporated milk.

Gradually add milk mixture to sauteed vegetables, stirring constantly to avoid lumps.

Whisk in egg yolks and seasonings (salt and pepper).

Cook the sauce for 5 minutes, stirring continuously, until it thickens.

In a mixing bowl, combine crabmeat (or crabmeat and shrimp) with the cooked sauce.

Pour the crab mixture into a greased baking dish (8-inch square or 9-inch round Pyrex).

Sprinkle grated Cheddar cheese evenly over the top.

Bake in a preheated oven at 350°F (175°C) for 12 to 15 minutes, or until golden brown and bubbly.

Let cool slightly before serving.