Gather all ingredients: bruise the lemongrass stalks, mince the garlic, slice the ginger, and prepare the chicken, vegetables, and herbs.
In a large heavy-bottomed pot, add the bruised lemongrass, minced garlic, and sliced ginger, then turn the heat to medium. Sauté for about 2 minutes until fragrant, and the mixture begins to bubble gently.
Pour in the chicken stock, then stir in fish sauce and freshly squeezed lime juice. Bring the broth to a gentle simmer, watching for small bubbles to form around the edges.
Add the chicken pieces and sliced chili to the broth. Cook for about 10 minutes, or until the chicken is just tender and cooked through, with the broth smelling vibrant and spicy.
Stir in the sliced mushrooms and cook for another 3-4 minutes, just until they soften but still hold some texture.
Drop in the fresh spinach and cook for 1-2 minutes until wilted but still bright green.
Taste the soup and adjust the seasoning with additional lime juice, fish sauce, or chili if desired for a balanced, lively flavor.
Remove the lemongrass stalks, then stir in chopped cilantro and torn Thai basil for fresh, fragrant finishing touches.
Give the soup a final stir, then ladle into bowls, ensuring each serving has a good mix of herbs, vegetables, and tender chicken.
Serve hot, garnished with extra herbs or chili slices if desired, and enjoy the vibrant, aromatic flavors of your Thai chicken soup.