Gather all your ingredients, including lemongrass, galangal, chicken, and herbs, ready for a quick and flavorful cooking process.
Heat the large pot over medium heat, then add a splash of oil and sauté the sliced garlic and ginger until fragrant, about 2 minutes, until they release a bright aroma.
Add the thinly sliced chicken to the pot, cooking until just browned and cooked through, about 3-4 minutes. Remove the chicken and set aside.
In the same pot, add the sliced lemongrass and galangal, cooking for another 2 minutes until they release a citrusy and piney aroma.
Pour in the chicken broth, bringing the mixture to a gentle simmer. Stir in fish sauce and chili paste, letting the broth bubble softly for about 10 minutes, allowing flavors to meld.
Return the cooked chicken to the pot, then add sliced mushrooms. Cook for another 5 minutes until the mushrooms are tender and the chicken is heated through.
Turn off the heat and stir in lime juice for brightness. Taste the broth and adjust seasoning with more fish sauce or lime if needed.
Serve the soup hot, garnished with sliced green onions and chopped cilantro for a fresh finish.