Thai Coconut Chicken Soup

Heat the vegetable oil in a large pot over medium heat, then add the ginger, garlic, and lemongrass and stir for 1-2 minutes until very fragrant.
Pour in the chicken broth and coconut milk, stirring to combine, and bring to a gentle simmer; do not let it boil hard, or the coconut milk can separate and look curdled.
Add the sliced chicken thighs and mushrooms, and simmer gently, uncovered, for 12-15 minutes, stirring occasionally.
Check that the chicken is fully cooked by cutting a piece in half; it should be white all the way through with no pink, reaching an internal temperature of 165F.
Stir in the fish sauce and brown sugar, and simmer for 2 more minutes to let the flavors blend.
Remove the pot from the heat and stir in the lime juice; adding it off the heat keeps the flavor bright and fresh rather than bitter.
Taste the broth and adjust with more fish sauce for saltiness or more lime juice for tang, until it tastes balanced between salty, sour, and rich.
Fish out and discard the large lemongrass pieces before serving.
Ladle into bowls, top with sliced chili if using and a generous amount of fresh cilantro.

Keep the heat gentle once the coconut milk is added; a rolling boil can cause the fat in coconut milk to separate and look grainy instead of smooth.

Add lime juice at the very end, off the heat, since prolonged cooking mutes its bright, fresh flavor.