Boil the rice noodles in boiling water for 3 minutes, then rinse with cold water.
Pour soy milk into a pot and heat over low heat.
Wash and thinly slice the onion, bean sprouts, and bok choy.
Add the cooked noodles and prepared vegetables to the heated soy milk.
Season with salt and pepper, then bring to a boil once more.
Serve in a bowl, garnished with cilantro and a squeeze of lemon.