Toast the sesame seeds in a wok or frying pan over a gentle heat until golden, then set aside. Wash and dry the lettuce, cut into chopstickable pieces and pile up on a serving dish.
Place the soy sauce and sugar in a bowl with two tablespoons stock or water and stir to dissolve the sugar. Tip the sesame paste into another bowl with a little oil from the jar and smooth it with a spoon. Stir in the soy sauce mixture in a few stages, making sure each addition is emulsified into the sauce before adding more.
When you have a smooth sauce, stir in the sesame oil and chilli oil, if using, and then, if you need it, add another tablespoon or so of stock or water until you have a sleek liquid with the consistency of single cream: it’s important that the sauce is thick enough to cling to the lettuce, but thin enough to pour. Add a little salt, to taste, but take care not to overdo it, because this dish is best enjoyed as a refreshing contrast to more strongly flavoured dishes.
Just before serving, pour the sauce over the lettuce, and garnish with the sesame seeds.
‘Lettuce captured alive’ variation: Dilute 15g sesame paste with one tablespoon of water and stir until smooth. Add two teaspoons light soy sauce, two teaspoons caster sugar, one and a half teaspoons Chinkiang vinegar and a quarter of a teaspoon of ground roasted Sichuan pepper. Stir in one teaspoon sesame oil, three to four tablespoons of chilli oil with sediment, and a couple of good pinches of salt, to taste. Use this to dress the lettuce, and omit the sesame seeds. With some versions of this dish, chefs will sizzle chillies and whole Sichuan pepper in oil, pour the hot oil over some chopped garlic to bring out its fragrance, and use this spicy oil instead of regular chilli oil.