Cheese Covered Halibut

2 pounds halibut
1/2 cup mayonnaise
1/2 cup sour cream
1/2 cup shredded cheddar cheese
1 tablespoon minced onion
2 teaspoons flour
1 1/2 teaspoons lemon juice
1/8 teaspoon cayenne pepper

Mix together the mayonnaise, sour cream, flour, lemon juice, onion and pepper in a small bowl. Arrange fish in baking dish in a single layer. Brush the fish with the mayonnaise mixture. Bake at 425 degrees for about 25 minutes. Sprinkle cheese over fish and cook 2 more minutes, or until cheese melts.