Pour stock into a saucepan, add the lemongrass, ginger, kaffir leaves, and bring to a boil. Reduce to a simmer, place lid on the saucepan and cook for 15 minutes.
Strain the soup, discard solids, and pour soup back into the saucepan.
Whisk in the coconut milk and bring back to a simmer. Add the onions and mushrooms and cook over medium heat for about 5-10 minutes, or until the onions are soft.
Remove saucepan from heat and whisk in the lime juice, fish sauce, sugar, and chili.
Garnish with cilantro and lime wedges. Serve over rice if desired.