Place potatoes and garlic into a medium saucepan, cover with cold water, add salt and simmer until potatoes are tender.
Drain, return the potatoes to the warm pot, place over low heat and allow the potatoes to steam dry.
Mash potatoes and garlic thoroughly with a masher or pass through a mouli and stir through milk.
Return the mashed potatoes to the saucepan, set over low heat and gradually add the cheeses, stirring through with a wooden spoon.
Melt the butter in a small frypan, add the sage leaves and continue to cook until the butter is nut brow and the sage is crisp.
Serve the mash with the sage brown butter poured over.