Rinse mushrooms, immerse in hot water, and soak until soft, about 15 minutes. Holding under water, squeeze mushrooms to release grit, then lift mushrooms out and squeeze dry. Cut off and discard stems. Cut mushroom caps into 1/4-inch-wide strips.
In a 5- to 6-quart pan, bring 3 quarts water to a boil over high heat. Add noodles and cook, stirring occasionally, until barely tender to bite, 4 to 6 minutes. Drain well.
Meanwhile, place a wok or 5- to 6-quart pan over high heat. When pan is hot, add salad oil and tilt pan to coat. Add garlic and pork; stir-fry until pork is lightly browned, about 4 minutes.
Add cabbage and stir just until it begins to wilt, 1 to 2 minutes.
Add noodles, mushrooms, soy sauce, vinegar, sesame oil, and pepper. Stir until noodles are hot, about 3 minutes. Transfer to plates. Sprinkle with onions.