Soak the dried soybeans in water for at least 3 hours or overnight. Afterwards, drain them and heat 2 tablespoons of vegetable oil in a wok over medium heat. Add the soybeans and stir-fry for about 5-7 minutes until golden brown and fragrant. Set aside to cool.

Bring a large pot of water to a rolling boil, then add the dried sweet potato noodles. Cook for about 5-7 minutes until chewy but not overly soft. Drain and rinse under cold water to stop the cooking.

In a separate pot, heat 4 cups of low sodium stock or boiling water over medium heat until gently simmering, about 5 minutes.

In a mixing bowl, combine 1 tablespoon of minced ginger, 3 cloves of minced garlic, 1 tablespoon of toasted sesame seeds, and 1-2 teaspoons of Sichuan chili flakes. Heat 2 tablespoons of vegetable oil in a small saucepan until shimmering, then pour the hot oil over the spice mixture and stir quickly to combine.

Mix in 2 tablespoons of Chinese black vinegar, 1 tablespoon of light soy sauce, 1 tablespoon of dark soy sauce, 1 teaspoon of sugar, 1 teaspoon of Sichuan peppercorn powder, 1 teaspoon of white pepper, 1 tablespoon of sesame oil, and 1 tablespoon of chili oil. Pour in the warm stock and mix well.

In a serving bowl, add a portion of cooked sweet potato noodles, ladle the soup base over, and top with pickled mustard stems, scallions, cilantro, and roasted soybeans or peanuts. Adjust seasoning to taste.