Rinse tuna, pat dry and season with salt and pepper. Sprinkle with soy sauce and let rest in a cool place for 30 minutes.
Heat sesame oil in a wok, pat tuna dry, place tuna in wok and fry for 4 minutes on each side. Wrap in foil and let rest for a few minutes. Arrange lettuce leaves on a plate. Cut fish fillets into thick slices and serve on lettuce leaves, sprinkle with sea salt and freshly ground pepper and add a little wasabi paste.