1. In 10-inch skillet over medium heat, in 2 tablespoons hot salad oil, cook onion until tender, stirring occasionally. With slotted spoon, remove onion to plate; set aside.
2. Meanwhile, let frozen fish stand at room temperature 15 minutes to thaw slightly. In pie plate, beat egg. On waxed paper, mix bread crumbs, salt, and dill weed.
3. Cut block of frozen fish crosswise into 4 pieces. Dip fish pieces in egg, then coat with bread-crumb mixture.
4. In drippings remaining in skillet, over medium heat, cook fish 10 minutes, turning once and adding 1 more tablespoon oil if necessary. Return onion to skillet. Stir in spaghetti sauce, water, and sugar; over high heat, heat to boiling. Reduce heat to low; simmer 5 minutes or until fish flakes easily when tested with a fork. Top fish with cheese slices. Cover skillet; cook 1 minute longer or until cheese melts.