Chantilly mashed potato casserole

Peel and quarter four russet potatoes of the same size, so the pieces cook evenly.

Put in a pot of cold water covering them by at least two inches; add two tablespoons of kosher salt.

Bring to a boil on high, then drop to medium and simmer until very tender but not falling apart, testing with a knife in the thickest part.

Drain well for at least five minutes.

Transfer to a large bowl and, while steaming hot, mash smooth with a wire masher until there are no lumps.

Add six tablespoons of butter, a little more salt and half a cup of grated Gruyere; mash until incorporated.

Add a few shakes of cayenne, finely grated Parmigiano Reggiano and about half a cup of buttermilk; mix with the masher.

Taste for salt now and adjust, mixing with a spatula; the Parmesan topping will season the cream.

Whip cold heavy cream by hand to fairly stiff peaks, a minute or two.

Cut half the whipped cream into the potatoes with the side of the spatula to lighten them, then carefully fold in the rest, keeping as much volume as possible.

Transfer to a well buttered casserole dish set on a sheet pan; spread evenly with the tip of a fork and smooth the top with a spoon.

Top with a generous grating of Parmigiano Reggiano and a very light dusting of cayenne.

Bake in the center of a 475 oven for about 15 to 20 minutes, until browned.

Optionally broil a minute or two for extra browning, watching so it does not burn.

Remove the sheet pan from underneath and sprinkle with sliced green onions.