Rinse the mackerel fillets and pat dry. Remove any stray bones with tweezers, and place the fillets in a large shallow dish.
Whisk together the lemon juice, olive oil and crushed garlic with a pinch of salt and pepper. Pour the mixture over the fish and turn the mackerel fillets to coat. Cover the dish and leave to marinate for 15-20 minutes.
Heat a ridged griddle pan over a medium-high heat. Cook the fish fillets in batches for 2-3 minutes on each side. Cook the skin-side first, and take care when turning to ensure that the skin is released from the pan and does not stick.
Transfer the fish to a serving dish. You may want to keep the grilled mackerel warm in a low oven while you cook the remaining fillets.
Serve sprinkled with chopped parsley, and with wedges of lemon to squeeze over.
