Steps
1. Preheat the grill to medium-high heat.
2. In a small bowl, mix together the softened butter, chopped parsley, lemon zest, salt, and pepper.
3. Place the mackerel fillet on a piece of aluminum foil and spread the herb butter mixture on top.
4. Wrap the foil around the fish to create a packet.
5. Grill the mackerel packet for about 8-10 minutes, or until the fish is cooked through.
6. Serve the grilled mackerel with lemon slices on the side.