Preheat the oven to 220°C (450°F).
In a small bowl, whisk together the melted butter and minced garlic.
Line a large baking sheet with aluminum foil, leaving plenty of overhang, and lightly coat the foil with cooking spray.
Pat the keta salmon dry and place it skin-side down on the prepared baking sheet.
Brush the garlic butter over the top of the salmon, then sprinkle evenly with salt, pepper, and chopped parsley. Arrange lemon slices around the salmon.
Fold the foil over the salmon to enclose it and bake for 17–20 minutes, until the salmon is opaque and flakes easily with a fork.
Carefully open the foil, sprinkle with additional parsley if desired, and serve with the roasted lemon slices.