For the salad dressing, remove the peel from a small piece of ginger and grate finely. Remove the peel from watermelon and cut the flesh into small pieces. Mix with ginger. Peel onion and half a clove of garlic, dice and add. Mix with sesame oil, Tabasco, soy sauce, brown sugar, chopped chili, fish sauce and lime juice and mash. Strain. Season heavily with salt and freshly ground pepper.
Rinse and chop the Chinese cabbage, pak choi, baby corn on the cob and scallions. Mix with kaffir leaves cut into strips, lemongrass stalks, half a clove of garlic, four bamboo stalks, thinly sliced Thai mint, cilantro, basil and bean sprouts.
Place tuna fillet with about 100 milliliters of soy sauce and a little finely grated ginger in a freezer bag and marinate for at least three hours.
Lightly toast the light sesame seeds in a frying pan and place on a flat plate. Mix with the dark sesame seeds. Remove fish from marinade, drain and fry in a little oil in hot frying pan for one minute on each side. Roll in the sesame mixture all over.
Arrange the lettuce in the center of the plate. Pour on salad dressing.
Cut tuna into thin slices and spread evenly on the lettuce in a fan shape.