Pour 2 1/2 to 3 quarts boiling water into a large bowl. Add noodles and let stand until soft and pliable, 5 to 15 minutes.
Meanwhile, peel and chop onion. Rinse and drain spinach and bean sprouts. In a small bowl, mix 3/4 cup broth, vinegar, 3 tablespoons soy sauce, and pepper.
Pour noodles into a colander and drain. If desired, cut noodles into shorter lengths with scissors. Set a 14-inch wok or 12-inch frying pan with 2-inch-tall sides (see notes) over high heat. When hot, add oil and swirl to coat bottom. Crumble pork into pan and stir until lightly browned, 1 to 2 minutes. Add onion, ginger, and garlic; stir often until onion begins to brown, 1 to 2 minutes.
Add spinach, noodles, and broth mixture. Mix until noodles are tender to bite and hot, about 3 minutes. If more liquid is desired, stir in 2 to 4 more tablespoons broth. Add bean sprouts and peanuts; mix and add more soy sauce to taste. Pour into a wide serving bowl.