Place potatoes in large pot of cold water to cover and add 4 tablespoons of salt to pot. Slowly bring to a boil and then reduce the heat and simmer until tender. Drain and let cool. Refrigerate potatoes overnight.
The following day, peel potatoes and grate into a bowl. Season, to taste, with salt and pepper and set aside. Heat 2 tablespoons of clarified butter in a medium saute pan over medium-high heat. Add 1 tablespoon of the unsalted butter and melt. Add leeks and saute until tender, about 5 minutes. Add crumbled bacon and set aside.
For the first Rosti:
Preheat the oven to 200 degrees F.
Add half of the remaining clarified butter and unsalted butter to a small or medium non-stick saute pan and heat over medium-high heat. Add 1/4 of the potatoes and 1/2 the leeks and spread evenly over the top. Cover evenly with 1/4 more of the potatoes. Brown the potatoes for about 5 minutes and then place a plate over the top. Flip the potatoes over, and slide them back into the pan. Brown for another 5 minutes. Sprinkle 1/2 of the Gruyere over the top and melt. Remove from the pan and place in the oven to keep warm.
For the second Rosti:
Add remaining clarified butter and unsalted butter to the saute pan. Repeat the process with the remaining potatoes, leeks, and Gruyere. Serve immediately.
The potatoes need to sit overnight in the refrigerator, so plan ahead.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.