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.