Cook the rice according to package directions. Cover and keep warm.
Meanwhile, in a large skillet with a lid, bring the coconut milk, fish sauce, lime juice, lemongrass and garlic to a boil over medium-high; reduce the poaching liquid to a simmer.
Season the chicken with salt; add to the poaching liquid. Cook, turning once, adding the mushrooms and scallion pieces and covering halfway through, until the chicken is cooked through, 10 to 12 minutes.Transfer the chicken to a plate; tent with foil. Simmer the liquid until thickened slightly, about 2 minutes.
Slice the chicken; serve over the rice and top with the sauce. Garnish with the sliced scallions and chiles, if using.
Clark’s new book, Dinner: Changing the Game, is filled with more than 250 unfussy, delicious recipes like this one. It comes out March 7.