Cut the tuna into the finest small slices. Sort out the pak choi, wash, dry and cut into strips, roast the sesame seeds dry in a pan. Mix the soy sauce, the lime juice, the sesame oil and the fish sauce into a salad dressing and season with chili and Szechuan pepper.
Arrange the pak choi on plates, pour the fine, raw tuna slices over them, drizzle with the dressing and sprinkle with the sesame seeds. Serve immediately.