1Heat oven to 350º. Cook and drain spaghetti as directed on package. Mix scallops, water and lemon juice in 1 1/2-quart saucepan. Heat to boiling; reduce heat to low. Simmer uncovered 1 to 3 minutes or until scallops are white. Remove scallops from saucepan; reserve 1/2 cup liquid.
2Melt margarine in same saucepan over medium heat. Cook mushrooms and onions in margarine, stirring occasionally, until onions are crisp-tender. Stir in flour, mustard, salt, pepper and paprika. Cook, stirring constantly, until bubbly; remove from heat. Stir in milk and reserved liquid. Heat to boiling, stirring constantly. Boil and stir 1 minute. Stir in sherry and 1/4 cup of the cheese.
3Mix spaghetti, scallops and sauce in ungreased rectangular baking dish, 11x7x1 1/2 inches. Sprinkle with remaining 1/4 cup cheese. Bake uncovered 25 to 30 minutes or until hot in center.