Meanwhile, bring the stock to a boil in a large pot. Add noodles upon boiling, cook for about 3 minutes. Drain and set aside both stock and noodles.
In a large wok, heat oil over medium-low heat. Sauté the onion and garlic. When the onion is transparent, add the mushrooms and tofu. Season with soy sauce, fish sauce, and pepper.
Add vegetables, stirring occasionally. Add a little extra oil or reserved stock to avoid sticking. When the vegetables are crisp yet tender, add all the noodles and the stock one cup at a time, taking care to not let the noodles get too soggy. Stir well until well incorporated.
Garnish with cilantro and serve with a squeeze of lemon.
Note on the fish sauce: Fish sauce is not vegetarian, but I have a recipe for a Vegan Fish Sauce substitute that is perfect for this recipe. If you are simply looking for a meatless pancit bihon, feel free to use regular fish sauce in this recipe.