Step 1

In a medium pot or Dutch oven, heat half the oil over medium-high heat. Add the chicken, stirring occasionally until the chicken is just cooked through. Remove from the heat, remove chicken from the pot. Return the pot to medium heat and add the remaining oil. Add the onion and saute for a few minutes until starting to soften.

Step 2

Add the chicken broth, mushrooms, kaffir lime leaves, galangal, lemongrass, chile, fish sauce, Nam Prik Pao, and sugar. Cover, bring to a boil, then reduce the heat and simmer for about 10 to 15 minutes. Add the cooked chicken, and tomatoes and cook another 1 to 2 minutes until the tomatoes are cooked but not falling apart.

Step 3

Remove from the heat and stir in the lime juice and cilantro leaves, if desired. Adjust seasoning as needed with salt. Serve immediately, discarding the lime leaves, galangal and lemongrass pieces either directly from the soup pot or from each bowl as you eat.