Place potatoes in a large saucepan; cover with cold water and add salt to water if desired. Bring to a boil over high heat. Reduce heat; simmer uncovered 14 minutes or until potatoes are very tender. Drain potatoes in a colander.
Melt 1 tablespoon of the butter in the same saucepan over medium heat. Add garlic; cook 1 minute. Add sage; cook 1 minute. Add cream; bring to a boil.
Put hot cooked potatoes through a ricer or press through a strainer.* Gradually add the potatoes to the cream mixture, beating with a wooden spoon or large whisk. Gradually beat in remaining butter 1 tablespoon at a time, stirring vigorously after each addition.
Stir in cheese until melted. Season with salt and freshly ground black pepper to taste.