Combine melted butter, oil, lemon juice, dill, lemon peel and chili flakes to make a marinade. Place fish steaks on baking tray lined with baking paper and brush with half the marinade.
Grill under a pre-heated grill, 2 inches from heat, for 4 minutes. Turn the steaks, brush with remaining marinade, and grill again until fish flakes to fork (about 4 more minutes).
Season with salt and pepper to taste and serve immediately with lemon wedges and your favourite side dish.