How to Make It
In a large pot, cover the potatoes and garlic with cold water and bring to a boil. Simmer over moderate heat until tender, about 20 minutes.
Drain the potatoes and garlic in a colander, shaking out the excess water. Add the half-and-half, butter and cheese to the pot and heat until melted; add the thyme. Remove from the heat. Press the potatoes and garlic through a ricer into the pot and season with salt. Stir over moderate heat until very hot. Transfer to a bowl, garnish with thyme sprigs and serve.