Preheat oven to 375˚F. Line a sheet pan with aluminum foil.
Layer lemon slices as wide and long as the filet of salmon. Place salmon on top, skin side down.
In a small bowl, whisk together butter, lemon juice, garlic, cayenne, and onion powder. Top salmon with 3/4 of the mixture. Sprinkled with salt and pepper evenly.
Wrap up foil around the salmon, making a sealed packet.
Bake for 15 minutes. Remove from oven and unfold foil to expose the top of the salmon.
Set broiler to High.
Spoon remaining butter over the top of the salmon.
Return to oven and broil for 5 to 10 minutes, until salmon is golden and cooked to desired doneness.
Garnish with parsley and serve.