Step 1

Preheat the oven to 350°F.

Step 2

In a medium sauté pan or skillet over low heat, melt 1 tablespoon of the butter. Add the minced garlic and diced shallot and cook for 1 to 2 minutes, or until fragrant. Add the flour to the pan to coat the garlic and shallots, and cook the mixture for a minute. Then pour in the milk and heavy cream while whisking to incorporate the garlic and shallots. Season the mixture with the chili crisp, grated Gruyère cheese, chopped thyme leaves, salt, and pepper. Increase the heat to medium-low and simmer until the sauce is slightly thickened but still runny, 8 to 10 minutes, stirring occasionally. Set aside.

Step 3

Lightly grease all sides of an 8 by 8-inch baking dish or casserole (6 cup) with the remaining 1 tablespoon of butter.

Step 4

Thinly slice the potatoes, about 1/8-inch thick, with the peel on, preferably using a mandoline. If you want, you can peel the potatoes, but keeping the skin on adds a nice texture when cooked. Add a ladleful of the prepared cream sauce to the bottom of the dish. Then, arrange the potato slices vertically in the dish in overlapping rows, followed by a spoonful of sauce to coat the potatoes. Make sure each layer of stacked potatoes is touching the others. The cream sauce will hold the potatoes together, so add generous spoonfuls after each stack. Fill the dish by using smaller pieces of potato to fill in any gaps between stacks.

Step 5

Pour the remaining chili crisp– cream mixture over the potatoes. Cover the dish with foil. Bake for 1 hour, or until all the potato slices are fork-tender.

Step 6

When the potatoes are all cooked, remove the foil and sprinkle the grated Parmesan over the potatoes. Place it under the broiler, uncovered, for 2 to 3 minutes, or until the cheese is melted and the top is golden brown. Be sure to keep an eye on the dish to prevent burning.

Step 7

Let it sit for at least 10 minutes before serving. Garnish with the green onions and serve.