Toast the sesame seeds in a small, dry pan and set aside. Wash the coriander, pat dry and chop finely.
Squeeze the citrus fruits and reduce the captured juice by half in a small saucepan over high heat. Then put the boiled down juice in a bowl and mix with red wine vinegar, soy sauce, cayenne and ginger. Gradually stir in the olive oil. Possibly season with a little sugar.
Cut the tuna fillet into thin slices, arrange on plates and drizzle with the citrus-ginger dressing. Serve sprinkled with the chopped coriander and toasted sesame seeds.