Cook the rice noodles according to package instructions. Drain and set aside.
In a large pot, heat the vegetable oil over medium heat. Add the diced onion and sauté until translucent, about 5 minutes.
Stir in the minced garlic and grated ginger, cooking for an additional 1-2 minutes until fragrant.
Pour in the vegetable broth, soy sauce, sesame oil, and red pepper flakes (if using). Bring to a boil.
Add the chopped bok choy and sliced mushrooms to the pot. Reduce heat and let simmer for 5-7 minutes until the bok choy is tender.
Add the cooked rice noodles to the soup and stir to combine. Heat through for another 2-3 minutes.
Serve hot, garnished with sliced green onions and fresh cilantro. Offer lime wedges on the side for squeezing over the soup.