Melt butter in a medium skillet over medium-high heat. Add onion; sauté 5 minutes. Stir in vinegar and thyme; cover and cook 5 minutes, stirring occasionally.
While onion cooks, combine flour, salt, garlic powder, and pepper in a shallow dish. Dredge fish in flour mixture. Heat a large nonstick skillet coated with cooking spray over medium-high heat. Add fish; cook 3 minutes on each side or until desired degree of doneness.
Spread 1 teaspoon mustard over top half of each bun. Layer 1 lettuce leaf, 1 fish steak, 1 tomato slice, and 1/4 cup onion mixture on bottom half of each bun. Top with top halves of buns.
very good sandwich. have made many times. company loved it and is now making it also !