Smash the lemongrass stalks with the flat side of a knife and cut into 2–3 pieces each. Peel and thinly slice the galangal (or ginger). Peel the shallots and halve them. Lightly bruise the kaffir lime leaves by stacking and folding them a few times. Leave the chiles whole (pierce once if you want extra heat).

In a large pot, combine the chicken stock and water. Add the prepared lemongrass, galangal/ginger, shallots, kaffir lime leaves, and chiles. Bring to a gentle boil over medium-high heat, then reduce to a simmer. Simmer uncovered for 15–20 minutes to infuse the broth.

After simmering, taste the broth for intensity. If very strong, remove after 15 minutes; if milder, simmer up to 25 minutes. Strain out and discard the aromatics, returning clear broth to the pot.

Primary proteins & vegetables

If using chicken, slice thighs thinly against the grain. Add the sliced chicken to the simmering strained broth and cook gently for 4–6 minutes, until just cooked through.

Add the sliced mushrooms and halved cherry tomatoes to the pot. Simmer for an additional 2–3 minutes until mushrooms are tender but still hold shape and tomatoes have softened slightly.

Seasoning & aromatics

Lower the heat to gentle. Stir in the coconut milk, if using, and heat through (do not boil vigorously once coconut milk is added).

Season the soup with fish sauce, fresh lime juice, and sugar. Start with the listed amounts, then adjust: add more fish sauce for saltiness, more lime for brightness, or a pinch more sugar to balance. Taste and balance until savory, sour, and slightly sweet.

Add the sliced green onions and roughly chopped cilantro to the pot and stir to combine. Heat just until herbs wilt, about 30 seconds to 1 minute.

Finish & garnish

Ladle the soup into bowls. Garnish each bowl with a lime wedge and a drizzle of sliced red chili or chili oil to taste. Serve immediately.