Bring a large saucepan of water to a boil. Salt the water and add the orzo. Cook until barely al dente, 8 to 10 minutes.
Drain the orzo and return it to the saucepan. Add the stock, lemon zest, thyme, 1/2 teaspoon salt and 1/4 teaspoon pepper. Cook over moderately high heat, stirring occasionally, until most of the liquid is absorbed. Stir in the butter and Parmesan just before serving.