Heat the avocado oil in a large pot or Dutch oven over medium heat. Add the onion and cook, stirring occasionally, until the edges begin to brown, about 5 to 8 minutes.
Add the garlic and mushrooms and continue sauteing another 3 minutes, until mushrooms begin to sweat.
Stir in the remaining ingredients, stir well, cover the pot, and bring to a full boil. Reduce the heat to a simmer and cook 20 minutes, or until the soup is very aromatic and the chicken is cooked through.
Remove and discard the lemongrass before serving.
Serve soup in big bowls with lime wedges and cilantro.
Notes
*Replace the ginger with galangal if you can find it!**Or 2 Tbsp lemongrass paste.***Replace the lime zest and juice with 10 kaffir lime leaves if you can find them.