mix oil, garlic, chili, sesame seeds and sprinkling of salt on a large flat dish. Rinse tuna and pat dry. Carefully press tuna into sesame seed (seeds should stick). turn and coat other side. cover and marinate in refrigerator for at least 1 hour.

meanwhile, cut radishes into matchsticks and put in bowl with half a lemon juice, to prevent browning.

heat a little extra oil pan frying pan over high heat. Sear tuna for 20 seconds of each side. Leave to rest for 5 minutes

put half a tuna steak on each of 4 serving plates and drizzle with remaining lemon juice. Drain radishes and scatter over with chives and extra sesame seeds if using