Chantilly Mashed Potato Casserole

1. Stage

Preheat the oven to 475 degrees F (245 degrees C).

2. Stage

Place potatoes into a pot, cover with fresh, cold water by 2 inches, and add 2 tablespoons kosher salt. Set heat to high and bring to a boil. Reduce heat to medium and simmer until tender, testing by poking potatoes with a knife. The time will depend on the size of your potatoes but could be between 15 and 20 minutes.

3. Stage

Remove from the heat and let drain in a colander for 5 to 10 minutes.

4. Stage

Transfer into a large bowl and mash thoroughly with a potato masher until no lumps remain. Add butter, kosher salt, and Gruyere cheese. Mix and mash until everything has been incorporated.

5. Stage

Add cayenne, 3/4 cup Parmigiano-Reggiano cheese, and buttermilk, and mash until well combined. Taste and adjust for salt.

6. Stage

Whisk cream in a metal bowl until thick and fairly stiff peaks form, 1 to 2 minutes. Pour 1/2 of the whipped cream onto the potatoes and cut it in with the side of a spatula to lighten the mixture. Repeat with remaining whipped cream but do it carefully to preserve the maximum amount of volume, cutting in until just barely mixed.

7. Stage

Transfer into a well-buttered casserole dish and distribute gently with a fork. Smooth the top with a spoon if desired. Sprinkle remaining 1/4 cup Parmigiano-Reggiano on top and dust lightly with cayenne.

8. Stage

Place the baking dish on a pan and bake in the center of the preheated oven until beautifully browned, 15 to 20 minutes.

9. Stage

Remove from the oven and sprinkle with green onions before serving. Chef John