Preheat the oven to 375°F/190°C, position a rack in the middle of the oven, and line a baking sheet with a large piece of foil. Spray it lightly with cooking spray.
To make the sauce, in a bowl, whisk together the melted butter, garlic, lemon juice, dill, red chili flakes, salt, and pepper.
Place the side of trout on the foil, and spoon the sauce over the top of the fish.
Fold the sides of the foil to cover and seal the foil packet, but not tightly so the top of the foil does not touch the sauce too much.
Bake in the oven for 15-20 minutes or until the thickest part of the fish is cooked through and can be easily flaked with a fork. The cooking time depends on the thickness of the fish.
Remove from the oven, and carefully open the foil (be careful of the hot steam!). Place under the broiler for a couple of minutes to lightly brown the top, then allow to rest before slicing and serve with chopped parsley and lemon slices.