Gather the ingredients.
Mix together cornstarch with soy sauce until well combined.
Marinate the chicken or tofu in the cornstarch-soy sauce mixture, coating all sides. Set aside.
Gently boil rice noodles 5 to 10 minutes just until noodles are soft enough to eat but still firm and a little bit crunchy (they will finish cooking later).
Drain and briefly rinse with cold water to keep from sticking. Set aside.
In a small bowl, combine soy sauce, dark soy sauce, fish sauce, 1 teaspoon sugar, lime juice, 1/4 cup stock, and chili sauce.
Stir well to dissolve sugar. Set aside.
Heat a wok or large frying pan over medium-high heat. Add 2 tablespoons oil and swirl around; then add the garlic, galangal or ginger, chicken or tofu (together with the marinade), mushrooms, and a few tablespoons of stock. Stir-fry 5 minutes. When pan becomes dry, add a little more chicken stock, 1 to 2 tablespoons at a time.
Add pepper and stir-fry another 1 to 2 minutes. Add the reserved noodles plus stir-fry sauce. Using two utensils, lift and turn the noodles or use a tossing motion, like tossing a salad. Keep the heat turned up. Stir-fry in this way until the sauce is well distributed throughout the noodles (1 to 2 minutes).
Add bean sprouts, continuing to stir-fry another minute.
Remove from heat and taste-test for salt and flavor, adding 1 tablespoon more fish sauce or soy sauce if not salty or flavorful enough. If too salty, add another squeeze of lime juice.
To serve, lift noodles out of the wok or frying pan and mound in a bowl or serving platter. Sprinkle with the sliced green onion and chopped coriander/cilantro. A wedge of fresh lime squeezed over just before eating also is delicious.