In a large saucepan or Dutch oven, combine coconut milk, water, lemon grass, shallots, and galangal.

Bring the mixture to a simmer over medium-low heat.

Simmer gently for 10-15 minutes to infuse the flavors.

While the soup simmers, prepare the remaining ingredients: slice the chicken, rinse and stem the chiles, and slice the mushrooms and cherry tomatoes.

After simmering the base for 10-15 minutes, add the lime juice, kaffir lime leaves, sliced chicken, Thai bird chiles, fish sauce, brown sugar, sliced mushrooms, and cherry tomatoes to the saucepan.

Continue to simmer until the chicken is fully cooked, approximately 10 minutes, depending on the thickness of the slices.

Taste and adjust seasonings as needed.

Serve hot, garnished with fresh cilantro (if available).