Thai Coconut Chicken Soup (Tom Kha Gai)

Prepare the tomato by cutting it into 8 wedges. Slice the galangal into ¼-inch disks and then cut the disks into half moons. Slice the lemongrass in half lengthwise and then into 2-inch sections. Juice the lime to yield 2 tablespoons.
Slice the chicken across the grain into ½-inch pieces.
Bring a 2.5-quart saucepan of water to a boil. Add the sliced chicken to the boiling water and poach it, stirring often, for 2 minutes. Remove the chicken with a slotted spoon or drain the whole pot over a mesh strainer. Set the poached chicken aside.
Rinse out the saucepan and wipe away any foam from poaching the chicken. Add the coconut milk, stock, galangal, makrut lime leaf, and lemongrass. Bring the mixture to a boil over high heat. Reduce the heat to maintain a quick simmer. Simmer 10 minutes.
Add the chicken, tomatoes, and mushrooms. Bring the soup back to a simmer again and cook for 5 minutes.
Remove soup from heat. Add lime juice, fish sauce, and brown sugar. Stir to combine and then taste to adjust flavor to your liking.
Ladle soup into bowls. Garnish with cilantro and chili oil.