Combine flour, 1/2 teaspoon salt, and 1/4 teaspoon pepper in a large mixing bowl, stirring well. Dredge fish in flour mixture; set aside. Heat 1/4 inch oil in a large skillet; add fish, and brown on both sides. Remove from heat; drain excess oil.
Place onion and potato slices over fish in skillet; set aside. Combine tomatoes, catsup, and remaining salt and pepper; pour over vegetables. Sprinkle with cheese; cover, and simmer 20 minutes or until onion is tender. Serve immediately from skillet.