Preheat broiler to high and position oven rack 4–6 inches from heat source
Lightly coat a baking sheet or broiler pan with cooking spray
In a small bowl, combine onion powder, paprika, garlic powder, ground black pepper, salt, and cayenne pepper
Pat haddock fillets dry with paper towels to remove excess moisture
Arrange haddock fillets in a single layer on the prepared baking sheet
Evenly sprinkle the spice mixture over both sides of the fillets
Melt butter gently and drizzle over the fillets
Place fillets under the broiler
Broil for 6–9 minutes, depending on fillet thickness, until fish flakes easily with a fork and surface is lightly golden
Remove the fillets from the oven and let rest for 1 minute
Transfer fillets to plates carefully using a fish spatula
Serve immediately with fresh lemon wedges on the side
Plating suggestion: Arrange fillets slightly overlapping on a pre-warmed plate, garnish with fresh lemon wedges and a light dusting of paprika for color
Serving suggestion: Pair with steamed green vegetables or a crisp salad for a balanced meal