Ingredients:
2 ½ pounds russet potatoes, peeled and thinly sliced 1/8-inch thick
2 shallots, thinly sliced
¼ cup fresh chives, finely chopped
Kosher salt
Black pepper, freshly ground
3 tablespoons butter
3 tablespoons all-purpose flour
1 clove garlic, minced
1 teaspoon ground mustard
¼ teaspoon dried thyme
¼ teaspoon paprika
2 cups whole milk (or 1 cup heavy cream, 1 cup milk)
1 cup white cheddar cheese, shredded
½ cup gruyere cheese, shredded
¼ cup Parmesan cheese, grated
Additional ¼ cup cheddar and gruyere cheese, shredded, for topping
2 tablespoons fresh parsley, finely chopped, for garnish

Directions:
Preheat oven to 400 degrees and grease a 2-quart baking dish with butter on bottom and all sides.

Use a mandolin slicer or food processor to slice the potatoes into 1/8-inch thick pieces; set aside.

Place butter in medium saucepan and melt over medium heat. Add the garlic and saute until aromatic, about 1 minute. Whisk in the flour and cook over medium heat for 1 minute.

Add the milk in a slow, steady stream, whisking constantly, until smooth and lump-free. Continue whisking over medium heat, until mixture is thick enough to coat the back of a spoon.

Remove from heat and add cheeses, ground mustard, thyme and paprika; stir until smooth. Taste and add salt and pepper as desired.

Place a layer of the sliced potatoes in the greased baking dish, one overlapping the other. Scatter the sliced shallots and chives over the potato layer, then top with 1/3 of the cheese sauce, spreading the thick sauce with a spatula or spoon, until the layer is entirely coated. Repeat for 2 more layers, then sprinkle a ¼ cup of cheddar and gruyere over the top layer of cheese sauce.

Spray a sheet of aluminum foil with cooking spray and cover the baking dish, sprayed side down. Place dish on a baking sheet and bake for 60 minutes, then remove foil and bake uncovered for another 25 to 30 minutes, until the cheese is fully melted and golden brown, and the potatoes are tender. For extra crispiness, broil on high for 1 to 2 minutes.

Garnish with a sprinkling of fresh parsley and let rest at room temperature for 5 minutes before serving.