Slice the flank steak into 1/4-inch thick pieces, about 2 inches long.
In a medium bowl, whisk the soy sauce, 1 tablespoon of the rice wine, the cornstarch, 1 teaspoon of the sesame oil, and brown sugar, until starch and sugar are dissolved.
Pour over the flank steak and turn to coat. Let stand for 20 minutes to 2 hours.
Meanwhile, bring 4 cups water to a boil, then remove from heat. Add the rice noodles, and stir occasionally, until they are tender. About 25 minutes.
Drain the noodles, rinse under cold water, drain again, and toss with the remaining 1 teaspoon sesame oil.
Next, make the sauce by whisking together in a small bowl the oyster sauce, black bean sauce, 1 tablespoon water, and the remaining 1 tablespoon rice wine. Set aside.
Heat a large wok (or skillet) over medium-high heat.
Add 1 tablespoon of the peanut oil and swirl to coat the bottom and sides of the wok.
Add the flank steak and the marinade. Cook until starting to brown, about 2 - 3 minutes.
Add the garlic and ginger, and stir-fry until steak is just no longer pink, about another 1 -2 minutes more.
Transfer the mixture to a platter and wipe out the wok with paper towels.
Return the wok to medium-high heat. Add the remaining 2 tablespoons peanut oil to the wok and swirl as before.
Add the onion and cook, without stirring, until browned on the edges, about 2 minutes.
Add the broccoli and scallions and stir-fry until the broccoli is crisp-tender, about 2 minutes.
Add the rice noodles and steak mixture and stir-fry until the noodles are hot, about another 1 - 2 minutes.
Add the sauce mixture and stir until the noodles are coated.
Transfer to a platter and serve.