Heat a large saucepan coated with cooking spray over medium-high heat. Add 3/4 cup onions; cook 2 minutes or until tender. Add sugar and next 6 ingredients (sugar through clam juice); bring to a boil. Cover, reduce heat, and simmer 15 minutes.
Increase heat to medium; add fish and shrimp. Cook 5 minutes or until shrimp are done (do not stir). Remove from heat; stir in 1/4 cup onions and butter.