Gather all your ingredients: smash the lemongrass stalks lightly to release their aroma, and slice the chilis for heat.
Place a large heavy-bottomed pot over medium heat, then add the smashed lemongrass and sliced chilis, sautéing for about 2 minutes until fragrant and slightly caramelized, filling your kitchen with a citrusy aroma.
Pour in the chicken stock and stir in the fish sauce, bringing the mixture to a gentle simmer. Let it bubble softly for 3-4 minutes to meld the flavors.
Add the bite-sized chicken pieces into the broth, cooking for about 8-10 minutes until tender and cooked through, with the broth turning slightly cloudy and fragrant.
Stir in the sliced mushrooms and cook for another 3-4 minutes until they soften but retain some bite, and the broth remains lively and aromatic.
Remove the pot from heat, then add lime juice and chopped spinach, stirring until the greens wilt and the broth brightens with a fresh acidity.
Taste the soup and adjust seasoning if needed—more fish sauce for saltiness, more lime for brightness, or chilis for heat.
Ladle the steaming, aromatic soup into bowls, then sprinkle generously with chopped cilantro and Thai basil for a burst of freshness and fragrance.
Serve immediately while bubbling hot, enjoying the vibrant flavors and tender chicken in every spoonful.
