Ingredients:
- 3 lbs Russet potatoes, peeled
- 4 tbsp unsalted butter
- 1/4 cup all-purpose flour
- 2 cups whole milk
- 1 cup heavy cream
- 3 cloves garlic, minced
- 1 tsp fresh thyme leaves
- 1/4 tsp nutmeg, freshly grated
- 1.5 cups sharp white cheddar, shredded
- 1/4 cup Parmigiano-Reggiano, finely grated
- 1 tsp fine sea salt
- 1/2 tsp freshly cracked black pepper
Instructions:
- Slice the potatoes. Peel the 3 lbs Russet potatoes and use a mandoline to slice them into uniform 1/8 inch rounds. Note: Do not soak these in water; the starch is essential for the sauce.
- Make the roux. In a saucepan over medium heat, melt 4 tbsp unsalted butter. Whisk in 1/4 cup all purpose flour and cook for 2 minutes until it smells nutty and looks like wet sand.
- Incorporate the dairy. Slowly whisk in 2 cups whole milk and 1 cup heavy cream. Note: Add the liquid in a thin stream to prevent any lumps from forming.
- Infuse the aromatics. Add 3 cloves minced garlic, 1 tsp thyme leaves, and 1/4 tsp freshly grated nutmeg. Simmer until the sauce is thick enough to coat the back of a spoon.
- Prepare the dish. Grease a 9x13 inch ceramic baking dish thoroughly with butter. Arrange one third of the potato slices in the dish, overlapping them slightly like shingles.
- Season and sauce. Sprinkle the layer with a portion of the 1 tsp salt and 1/2 tsp pepper. Pour over one third of the sauce, ensuring it seeps between the slices.
- Repeat the layers. Create two more layers with the remaining potatoes and sauce. Top the final layer with 1.5 cups shredded cheddar and 1/4 cup Parmigiano Reggiano.
- Cover and bake. Cover the dish tightly with foil. Bake at 350°F (175°C) for 45 minutes until the potatoes begin to soften and the sauce is bubbling.
- Finish the crust. Remove the foil and bake for another 30 minutes until the cheese is deep golden brown and the edges are crackling.
- Rest the dish. Let the potatoes sit for 10 minutes before serving. Note: This allows the sauce to set so the layers don't slide apart when sliced.