Cut steak into 1 1/2" cubes. Toss with minced garlic, ginger, and scallions. Let stand 15 minutes. Meanwhile, soak black mushrooms in 1/3 cup warm water until soft, about 10 minutes. Drain, reserving water for sauce. Remove and discard stems. Bring a medium saucepan of salted water to a boil. Add snow peas and cook until bright green, about 2 seconds. Drain and rinse under cold running water. In a small bowl, combine oyster sauce, soy sauce, sugar, cornstarch, and reserved water from mushrooms. Blend thoroughly. Set sauce aside. In a wok, heat oil over high heat until smoking. Add beef cubes and stir fry until lightly browned outside but still rare inside, 2 to 3 minutes. Add mushrooms, snow peas, and sauce mixture. Return to a boil and cook, tossing to mix well, until sauce thickens, 1-2 minutes. Remove from heat and serve.