In a medium saucepan, heat the oil on medium-high heat.
Add the chopped onions, chili peppers and garlic and cook until onions are translucent, about 5 minutes.
Add the sliced mushrooms and ginger to the pot and cook, stirring occasionally for 10 minutes.
Pour in the chicken broth.
Place the pieces of lemongrass on a cutting board and with the flat side of a knife, gently press down on the lemongrass to release the flavors. Add to the pot.
Bring to a boil, then reduce heat to medium-low. Simmer for 15 minutes.
Stir in the bok choy and the shrimp. Simmer until the shrimp is opaque, 3-4 minutes.
Taste and add a pinch or two of salt if necessary. Remove pot from heat and cover.
Prepare the thin rice noodles according to package directions. The rice noodles cook very quickly.
To serve, spoon the rice noodles into a bowl and pour the soup over the noodles. Top with chopped cilantro, if desired.