Season the tuna. Heat the oil in a large frying pan, add the tuna and cook for 3–5 minutes or until browned all over. Put the sesame seeds on a plate and press the seared tuna into them until well coated. Cook in a preheated oven, 220°C (425°F), Gas Mark 7, for 10–12 minutes until browned, but still pink inside.
Mix together the ingredients for the dressing. Cut the tuna into thick slices and arrange on serving plates with the cucumber slices, avocados and spring onions. Drizzle over the dressing to serve.