Start by chopping your fresh ingredients. Dice the cilantro, slice the mushrooms, and finely chop the serrano chili after removing the seeds. If you’re using rotisserie chicken, shred it into bite-sized pieces.
In a large pot, add your chicken stock and bring it to a gentle simmer over medium heat. Stir in the lemongrass paste, ginger paste, and kosher salt. Allow the mixture to simmer for about 5 minutes, letting those aromatic flavors infuse the broth.
Next, add the sliced mushrooms and chopped serrano chili to the pot. Let them cook for another 5 minutes until the mushrooms are tender.
Stir in the shredded chicken and coconut milk, mixing well. Allow the soup to come back to a gentle simmer. This step will create a creamy, comforting texture that’s characteristic of Tom Kha Gai.
Add the brown sugar and lime juice to the pot. Taste the soup and adjust seasoning if necessary, adding more salt or lime juice to suit your palate.
Just before serving, fold in the halved cherry tomatoes and chopped cilantro. This adds freshness and color to the soup, making it even more appealing.
Ladle the soup into serving bowls and garnish with additional cilantro. For those who enjoy some heat, a few drops of hot sauce can be added to taste.