Cook 1-1/2 cups elbow macaroni as directed on package. While it cooks, melt 2 tablespoons butter. Stir in 1-1/2 tablespoons flour and liquid from 1 can Chicken of the Sea Oyster Stew. Cook, stirring, until smooth. Add 1 cup diced process Cheddar cheese. Heat gently until cheese is melted. Mix with drained macaroni. Add 1 can tuna, coarsely flaked, and oysters from the stew. Heat and serve at once. Or pour into shallow 1-1/2 quart baking dish, dot top with thin tomato wedges and bake at 375 degrees (moderate) 15 to 20 min. Makes 4 servings.