Peel the potatoes (and other root vegetables, if using), and cut to half-inch cubes. Put in a saucepan then add the salt and the milk, which should almost cover. Bring it all to a boil, reduce heat and simmer until potatoes are very tender. Allow to cool slightly. Use a slotted spoon to transfer potatoes to a ricer, and rice into a pan that will serve as the top of a double boiler. When all the potatoes have been processed, leave them loose rather than stirring. Pour the liquid back into the potatoes, down the side of the pan. Cover and refrigerate until within an hour or two of serving time.

In the meantime, heat the heavy cream. When it comes to a simmer, turn it off and immediately add the sage and bay leaves. After the herbs have steeped in the cream for half an hour, remove and discard them (leaving as much of the cream as possible). Process the cream and roasted garlic together in a spice grinder, then transfer to a bowl along with the butter. Refrigerate.

In a small skillet, toast the coriander and peppercorns until they’re aromatic. Grind using a spice grinder, then mix the ground spices with the grated parmesan. Refrigerate.

Close to serving time: Place water in the bottom of your double boiler, then set up the refrigerated potatoes above them. Add the cream mixture. Bring the water in the bottom pan to a simmer, then continue simmering until the potatoes are hot. Add the parmesan mixture and stir everything together. If the potatoes are too thick, heat up the extra milk or cream and stir as much into the potatoes as is needed. Check seasoning and adjust if needed; serve.