Combine the sliced chicken with the cornstarch, and 2 teaspoons each of neutral oil and fish sauce. Set aside.

To a medium soup pot over medium-high heat, add the remaining 1 tablespoon of oil and the mushrooms. Fry the mushrooms for about 5 minutes, or until tender. Add the chicken stock, lemongrass, galangal, lime leaves or zest, chilies and red curry paste (if using), and sugar. Bring to a boil over medium-high heat. Cover, lower the heat, and simmer for 6 minutes.

Stir in the coconut milk, and bring back up to a simmer over medium-high heat—watch the pot and don’t let it come up to a rolling boil, or the coconut milk could split. Add the chicken. Bring back up to a simmer, lower the heat to medium-low, cover, and cook for 8 minutes, until the chicken is cooked through.

Turn off the heat, and stir in the remaining fish sauce, lime juice, and fresh cilantro (or scallions/green onions, if you're not a cilantro fan). Season with salt to taste if needed. Ladle into bowls, and serve on its own or with some steamed jasmine rice!