Preheat the oven to 475 degrees fahrenheit.
Spray a cookie sheet with cooking spray.
Rinse the haddock fillets, and pat dry.
Arrange the fillets in a single layer on the prepared cookie sheet, and spray them slightly with cooking spray.
In a small bowl, mix together the onion powder, paprika, garlic powder, ground black pepper, salt, and cayenne pepper.
Dip the fillets in the seasonings, coating them evenly.
Place the fillets on the prepared cookie sheet. Turn to disperse the seasoning evenly.
Bake the haddock for 10 minutes. Rotate the cookie sheet from front to back midway of the baking time.
During the final 5 minutes of baking, spread the butter over the fillets.
Remove the cookie sheet from the oven, and place the fillets and lemon wedges on serving plates.
Serving Suggestions: Lemon wedges add a nice flavor to this dish. Serve with crusty bread and a simple salad.