Rinse the mackerel fillets under cold water to remove scales and impurities. Check for bones and remove if necessary. Pat dry with paper towels.

In a mortar, combine coriander leaves, lemon zest, white wine vinegar, olive oil, salt, and pepper. Grind until smooth.

Coat the fillets with the marinade and refrigerate for at least 1 hour.

Prepare the radish pickle by combining grated radish, ginger, more coriander, nigella seeds, lemon zest, honey, and salt in a mixing bowl.

Preheat the grill until coals are white-hot, about 15-20 minutes.

Carefully thread two marinated fillets onto each skewer.

Grill the skewer for 10-15 minutes, turning regularly and basting with leftover marinade.

Serve the grilled fillets topped with radish pickle on a serving dish.