In a large sauté pan (with sides and a lid), whisk together the coconut milk, fish sauce, lime juice, lemongrass, and garlic. Bring to a boil.
Add the chicken and the mushrooms to the pan and simmer on low for 5 minutes. Flip over the pieces of chicken, cover the pan, and cook for 5 or 6 minutes more until the chicken is done (no longer pink) all the way through.
Pour the chicken mixture over the cooked rice, garnish with fresh cilantro, and serve with a side of green veggies.