- In a large pot, cook the potatoes in salted water until tender, about 20 minutes. Drain.
- Meanwhile, in a small saucepan, bring the milk, garlic, and rosemary to a boil. Let steep for about 10 minutes. Remove the rosemary sprig.
- With a masher, coarsely crush the potatoes with the butter. With an electric mixer, purée until smooth with the milk. Season with salt and pepper.
The puree is at its best when prepared at the last minute. You could still prepare it just before the guests arrive and leave in the pan on the stove, off the heat. Just before serving, reheat over low heat and add a little milk, if necessary.