For the beef and marinade:
1½ pounds flank steak, halved lengthwise then thinly sliced against grain on bias-cut
1 tablespoon soy sauce
3 tablespoons Shaoxing rice wine or dry sherry
½ teaspoon baking soda
About ½ teaspoon salt
About ⅓ cup cornstarch
4 tablespoons peanut or other neutral oil
For the sauce:
Zest of 2 blood oranges, Cara Cara oranges, or tangerines, plus ½ cup blood orange, Cara Cara orange or tangerine juice
3 tablespoons rice wine vinegar
2 tablespoons light soy sauce
2 tablespoons dark soy sauce
2 tablespoons light brown sugar
1 tablespoon cornstarch dissolved in a splash of water to form a slurry
1 tablespoon dark sesame oil
To stir-fry and to serve:
2 bunches broccolini, cut into bite-sized pieces
1 large or 2 small leeks, split lengthwise, cleaned and thinly sliced (whites and light greens)
2 inches ginger, peeled and cut into thin planks then thin strips
4 cloves garlic, thinly sliced or chopped
2 jalapeño peppers or serrano peppers, seeded and chopped or thinly sliced with seeds for spicy or 2 Fresno peppers for milder preparation
White rice, for serving
Toasted sesame seeds
For the beef and marinade, place meat in bowl or shallow dish, combine marinade (soy sauce through salt) and toss with beef in bowl, let stand 15 minutes. To cook the meat, coat evenly with cornstarch then cook in 2 batches, first in 2 tablespoons peanut oil, then add another tablespoon for second batch and remove beef to platter.
For the sauce, remove the zest from orange or tangerines with vegetable peeler first, then very thinly slice into strips and reserve—then juice the fruit. Combine orange or tangerine juice with the rest of the ingredients in a bowl and reserve.
For the stir-fry, place ½ cup water in large skillet with lid and bring to boil. Add broccolini and season with a little salt, cover 2 to 3 minutes, uncover and let water absorb, turn off heat.
Heat a large nonstick skillet over medium-high heat, add remaining 1 tablespoon oil from preparing meat, a turn of the pan, toss leeks a minute or 2, add ginger, garlic, chilies and reserved sliced zest, toss 1 minute, add sauce and beef and combine a minute or 2 more. Serve with white rice and steamed broccolini and sesame seeds sprinkled on top.