Melt the butter in a medium pot over medium heat. Add the orzo, thyme, shallots and garlic and cook until the orzo is golden brown and the shallots are softened, 3 to 4 minutes. Stir in the rice, then add the chicken stock. Bring to a boil, then reduce the heat and simmer, covered, for 18 minutes. Fluff with a fork and serve.