In a bowl, mix together 2 tbsp balsamic vinegar, 1 tsp brown sugar, packed and 1 tsp Dijon 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.