Preheat oven to 425°F.
In a 12- x 18-inch metal roasting pan or a 5-quart Dutch oven, heat the oil over medium heat. Season the chicken with salt and white pepper, then place it skin side down in the pan and sear for 3 minutes. Transfer the pan to the oven and roast until golden brown, about 5 minutes. Flip the chicken and roast for an additional 5 minutes. Remove the chicken from the oven and transfer to a plate.
Discard all but 1 tablespoon oil from the pan. Place the pan back over medium heat. Add the onion, garlic, and ginger and cook, stirring frequently, until the onion is translucent, 3 to 4 minutes. Add the mushrooms and continue cooking until softened, another 3 to 4 minutes. Add the rice, chicken stock, and coconut milk. Season with salt and white pepper and stir to combine.
Nestle the chicken and lemongrass on top of the rice. Increase the heat to bring the liquid to a boil. Cover the pan with aluminum foil, place it in the oven, and bake until the rice is tender and the chicken is cooked through, about 15 minutes.
Remove the pan from the oven and let the dish rest, covered, on the stovetop for an additional 10 minutes. Garnish with the cilantro and green onions. Serve immediately, straight from the pan, with lime wedges.
