Gather all your ingredients, prepping the lemongrass, galangal, garlic, and ginger by slicing thinly to release their aromas.
Heat the large heavy-bottomed pot over medium heat and add a splash of oil. Sauté the minced garlic and ginger until fragrant, about 2 minutes, until they become fragrant and slightly golden.
Add the sliced chicken to the pot and cook until just browned, about 3-4 minutes, stirring occasionally to prevent sticking.
Remove the chicken and set aside. In the same pot, add the sliced lemongrass and galangal, cooking for 2 minutes until they release a citrusy aroma and become slightly tender.
Pour in the chicken broth, bringing it to a gentle simmer. Stir in the fish sauce and chili paste, allowing the flavors to meld for about 10 minutes, with gentle bubbling.
Return the cooked chicken to the pot along with sliced mushrooms. Continue simmering for another 5 minutes until the mushrooms are tender and the chicken is cooked through.
Finish the soup by stirring in freshly squeezed lime juice, then taste and adjust seasonings if needed, adding more fish sauce or lime for brightness.
Serve the hot soup in bowls, garnished with chopped cilantro and sliced green onions for a burst of fresh color and flavor.