(1) Combine lemon juice, salt and paprika in a shallow dish. Add halibut, turning to coat. Chill 1 hour to marinate, turning once or twice.
(2) Preheat oven to 450 degrees.
(3) Saute onion in heated butter until soft.
(4) Place halibut in a sprayed 11x7 inch baking dish; top with onions.
(5) Bake 10 minutes or until cooked through.