Thai Chicken Coconut Soup

2 cups (500 ml) chicken stock
Six ¼-inch (6 mm) thick slices galangal or fresh ginger
2 stalks lemongrass, cut into 2 inch (5 cm) long pieces and bruised
4 makrut lime leaves, torn in half (optional)
1 tablespoon palm or brown sugar
1 ½ teaspoons Roasted Red Chile Paste or Nam Prik Pao (optional)
1 cup (250 ml) coconut milk
½ pound (250 g) boneless, skinless chicken thighs, cut into 1-inch (2.5 cm) pieces
1 cup (70 g) sliced white button mushrooms
3 tablespoons freshly squeezed lime juice
2 tablespoons ﬁsh sauce (nam pla)
2–3 fresh hot red or green chiles, preferably Thai, smashed
Fresh coriander leaves (cilantro) for garnish

Put the chicken stock, galangal, lemongrass, makrut lime leaves (if using) palm sugar, and roasted red chile paste (if using) in a medium saucepan and bring to a boil over medium heat.
Add the coconut milk, reduce the heat and bring to a simmer. Add the chicken and mushrooms and continue simmering until the chicken is white and opaque, 5–7 minutes.
Quickly stir in the lime juice and fish sauce. Float the chiles on the soup. Dish out into individual serving bowls and sprinkle with fresh coriander leaves. Serve immediately.