Tamari and sesame oil give this quick, easy stir-fry a nutty kick.
Heat a large skillet or wok over high. Add 2 tbsp. oil, two turns of the pan. When the oil smokes, add the beef and stir-fry until browned, about 2 minutes. Add the leeks; season with salt and pepper. Add the ginger and half the garlic and toss until the leeks soften, about 2 minutes more. Add the hoisin and the water if the beef and leeks seem dry. Transfer to a bowl.
Add the remaining 1 tbsp. oil, one turn of the pan, to the wok. When the oil smokes, add the lettuce and stir-fry just until wilted, about 1 minute. Add the remaining garlic and stir-fry until fragrant, about 1 minute more. Add the tamari and sesame oil; season with pepper. Transfer to a serving dish or dinner plates. Top the lettuce with the beef and leeks. Sprinkle with sesame seeds (if using).