Instructions

1. Cut steak in thirds along the grain. Freeze 15 minutes to firm slightly; thinly slice crosswise.
2. Sliver zest from enough tangerines (or clementines) to get 1 Tbsp; set aside. Squeeze enough of the fruit to get ½ cup juice. Cut bok choy stems into 1-inch pieces and the leaves into 2-inch pieces. Discard mushroom stems; thinly slice caps.
3. Whisk the juice, oyster sauce, soy sauce, vinegar, cornstarch and pepper in a small bowl. Place next to the stove.
4. Heat a 14-inch flat-bottom carbon-steel wok or large skillet over medium-high heat until very hot. Add 2 tsp oil and swirl to coat. Add half the beef in one layer; cook, without stirring, for 1 minute .
5. Then toss until the beef is no longer pink on the outside, 30 seconds more. Transfer to a plate. Repeat with 2 more teaspoons oil and the remaining beef; transfer to the plate.
6. Add 2 tsp oil to the pan; swirl to coat. Add the bok choy stems and mushrooms; stir-fry until the mushrooms release their liquid, about 3 minutes. Add the bok choy leaves; stir-fry until wilted, 1 to 2 minutes. Transfer the bok choy and mushrooms to the plate with the beef.
7. Add the remaining 1 tsp oil, ginger, garlic, scallion whites and chile to the pan and stir-fry for 30 seconds. Add the reserved sauce and heat until bubbling.
8. Return the beef, vegetables and any accumulated liquid to the pan; gently stir until heated through, about 1 minute more. Stir in scallion greens and the reserved zest.