In a bowl, mix together vinegar, brown sugar and mustard.

Rinse fish and pat dry with paper towels.

Lightly spray fish with cooking spray, then sprinkle with black pepper.

Heat a large non-stick skillet lightly coated with cooking spray over medium-high heat.

Add fish and cook until browned on the bottom, about 4 minutes.

Turn pieces over. Spoon vinegar mixture over browned sides.

Reduce heat and cook until fish is opaque but still moist in the center, about 4 to 5 minutes longer. Serve immediately.