Serves 8-10, prep time: 15m, total time: 2h
Russet or Idaho potatoes, washed, skin on (7-8 medium)
White onion (1 medium)
Gruyere cheese (16 oz or 2 cups)
Heavy whipping cream (2 and 1/4 cups)
Kosher salt (2 teaspoons, divided)
Black pepper, finely ground (1 teaspoon, divided)
Unsalted butter (1 tablespoon)
Preheat oven to 375°F and set rack to middle of oven. Rub the inside of a 9" x 13 "baking dish on all sides with the butter and then line with parchment paper, being sure to leave excess paper on each end of the dish so you can lift the potatoes out after cooking. Press the parchment paper firmly around all sides (keep in mind the corners will be crinkled and that's okay).
Now, set all of the remaining ingredients out (this will be an assembly line process, and one that should move relatively quickly so the potatoes don't oxidize and turn color). Using a grater or food processor, grate the cheese. Using a mandolin or food processor set to slice at 1/8" thickness, slice the onion and set aside. Pour 1/4 cup of heavy whipping cream in the baking dish and spread around the bottom of the baking dish with your fingers. Leaving the skin on, slice just enough potatoes (1 -2 ) to start your first layer of the dish. Slice the potato by cutting off each end so you have a flat surface on each side. Then, cut the potato in half so you have 2 round, flat halves. Now, slice the potato using a mandolin or food processor set to 1/8" thickness.
Layer the sliced potatoes in the bottom of the banking dish in an overlapping fashion so that they appear fanned and cover the bottom of the dish. Now, sprinkle the following in order over the potatoes: 1/4 cup of sliced onion, 1/3 cup of the grated cheese, 1/2 teaspoon of Kosher salt, and 1/4 teaspoon of black pepper. Pour 1/2 cup of the heaving whipping cream on top of the potatoes and with your hands, press down on the potatoes so that the potatoes are submerged in the cream. Now, repeat this process 2 more times. For the final layer, repeat the process, but add the cheese at the end, on top of the cream. Depending on the depth of your dish, the potatoes should be at least 1/2" from the top.
Cover with foil and bake the potatoes for 45 minutes, then remove the foil and cook for another hour, until the top of the potatoes are bubbly and a golden brown. Remove from the oven and cover with foil until ready to serve, the potatoes will stay warm for quite some time; or leave in the oven at 200°F until ready to serve.
To serve, carefully grab the excess parchment paper on each end and lift out onto a counter or cutting board. Now, cut and serve. Using parchment paper allows you to get all those crispy edges on the bottom and sides of the potatoes. If you wish to serve in the dish, simply omit using parchment paper altogether, but still butter your baking dish before layering your ingredients.
The potatoes can be made at least a day ahead of time and then re-heated in the oven at 350°F for 35-40 minutes.