Preheat oven to moderate (350°F.).

If sea scallops are used, cut them into quarters or eighths. Pick over the scallops to remove any small bits of shell.

Cook the onion, celery and green pepper in the butter until onion is translucent. Blend in the flour and salt and pepper to taste. Gradually add the milk, stirring constantly. When the mixture is thickened and smooth, add the scallops and remove from heat.

Arrange half of the rice, scallops and cheese in a well-buttered one-and-one-half-quart casserole, then repeat the layers, ending with a layer of cheese.

Bake the casserole until the mixture is thoroughly heated and cheese is golden brown.