Melt the butter in a medium saucepot over medium heat. Add the rice, onion and thyme and saute until the onion is softened, 4 to 5 minutes. Season with salt and pepper. Pour the chicken stock over the rice, cover and cook over medium-low until tender and fluffy, about 15 minutes. Fluff with a fork, fold in the parsley and let stand 5 minutes before serving.