Rinse the coriander, shake dry and chop finely. Peel and squeeze the garlic. Mix both with lemon juice and olive oil. Rinse the tuna steaks with cold water and pat dry with kitchen paper. Turn the steaks in the marinade and let them steep for about 1 hour.
Grill the tuna steaks for about 5 minutes on each side, brushing the marinade over and over again. Season with salt and pepper.