For the beef and sauce:
8 ounces flank steak, trimmed of excess fat and sinew
2 1/2 tablespoons Shaoxing wine or dry sherry
2 tablespoons soy sauce or tamari
1 1/2 teaspoons cornstarch
1 teaspoon vegetable oil
1 teaspoon rice vinegar
1 teaspoon packed dark brown sugar
1/2 teaspoon toasted sesame seed oil
For the stir-fry:
3 teaspoons vegetable oil, divided
8 ounces fresh brown or porcini mushrooms, halved or quartered if large
1 bunch asparagus (about 1 pound), ends trimmed and cut into 1-inch pieces
2 teaspoons peeled, minced fresh ginger
2 cloves garlic, finely chopped
1/4 cup water
1 teaspoon toasted sesame seeds (optional)
Steamed white rice or brown rice, for serving
For the beef and sauce: Cut the flank steak crosswise (with the grain) into 2- to 3-inch-wide strips. Cut each strip against the grain into 1/4-inch-thick pieces. Place the beef in a medium bowl; set aside.
Whisk the remaining ingredients in a small bowl until the cornstarch and sugar are fully dissolved. Transfer 1 tablespoon of this sauce mixture to the bowl of beef and toss to coat the meat; set the remaining sauce aside.
For the stir-fry: Heat a large frying pan or flat-bottomed wok over medium-high heat until a drop of water vaporizes immediately on contact. Drizzle in 1 teaspoon of the oil, then add the beef and spread into an even layer. Cook undisturbed until the beef begins to sear, about 1 minute. Using a metal spatula, stir-fry for 30 seconds until the beef is lightly browned, but not cooked through. Transfer to a large plate.
Drizzle 1 teaspoon of oil into the pan, add the mushrooms, and season with salt. Stir-fry until the mushrooms are browned and softened, about 2 minutes, then transfer to the plate with the beef.
Drizzle the remaining teaspoon of oil into the pan, add the asparagus, ginger, and garlic, and season with salt. Stir-fry until the garlic is fragrant, about 30 seconds. Add the water and stir-fry until the asparagus is crisp-tender and bright green, about 3 minutes.
Add the reserved beef, mushrooms, and any juices and stir to combine. Whisk the sauce again to recombine and dissolve the cornstarch, then add to the pan. Stir-fry until the beef is just cooked through and the sauce thickens and is glossy, about 1 minute. Transfer to a serving dish, sprinkle with the sesame seeds, and serve with rice.