Combine first 5 ingredients in a bowl. Stir in 1/4 teaspoon salt. Cover and chill.
Heat a large nonstick skillet over medium-high heat. Sprinkle both sides of fish with the remaining 1/2 teaspoon salt and black pepper. Coat pan with cooking spray. Add fish to pan; cook 3 minutes on each side or until fish flakes easily when tested with a fork or until desired degree of doneness. Place 1 fillet on each of 4 plates, and top each serving with 1 1/2 teaspoons butter mixture. Serve with lime wedges, if desired.