Sprinkle both sides of fish with salt and pepper. Place on a broiler pan coated with cooking spray. Spray fish with cooking spray. Broil 6 to 8 minutes or until fish flakes easily when tested with a fork.
Meanwhile, melt 1 tablespoon butter in a large nonstick skillet over medium heat. Add onion; cook 4 minutes or until tender, stirring constantly. Stir in chili powder and cumin; cook 30 seconds, stirring constantly. Remove from heat; stir in 2 tablespoons butter, water, and lime juice until well blended. Spoon onion mixture evenly over fish.