Ingredients

• 2 pound russet potatoes, thinly sliced
• 4 tablespoon unsalted butter
• 3 tablespoon all-purpose flour
• 2 cup whole milk
• 1 cup heavy cream
• 1.5 cup Gruyère cheese, shredded
• 0.5 teaspoon salt
• 0.25 teaspoon white pepper
• 0.25 teaspoon nutmeg, freshly grated
• 0.5 cup panko breadcrumbs
• 0.5 cup Parmesan cheese, finely grated
• 2 tablespoon unsalted butter, melted
• 1 teaspoon fresh thyme, leaves only

Steps

1. Preheat oven to 350°F (175°C). Butter the 9x13 inch baking dish and set aside.
2. Peel and thinly slice the potatoes into uniform 1/8-inch slices using a mandoline slicer or sharp knife. Place sliced potatoes in a large bowl with cold water to prevent browning.
3. In a large saucepan, melt 4 tablespoons butter over medium heat. Once melted and foaming, add flour and whisk constantly for 1 minute to create a roux, stirring to prevent lumps.
4. Gradually add milk and cream to the roux, whisking continuously to prevent lumps. Increase heat to medium-high and continue whisking until the sauce thickens and comes to a gentle boil, about 8 minutes.
5. Remove the sauce from heat. Stir in 1½ cups shredded Gruyère cheese, salt, white pepper, and grated nutmeg until the cheese is fully melted and incorporated.
6. Drain the sliced potatoes thoroughly and pat dry with paper towels.
7. Spread a thin layer of the cream sauce on the bottom of the prepared baking dish. Layer half the sliced potatoes in an overlapping pattern over the sauce.
8. Pour half of the remaining cream sauce over the potatoes, spreading evenly. Layer the remaining sliced potatoes on top in an overlapping pattern.
9. Pour the remaining cream sauce over the top layer of potatoes, ensuring even coverage.
10. In a small bowl, combine panko breadcrumbs, finely grated Parmesan cheese, and fresh thyme leaves. Drizzle with 2 tablespoons melted butter and toss until evenly moistened.
11. Sprinkle the breadcrumb-Parmesan mixture evenly over the top of the gratin.
12. Bake in the preheated 350°F oven for 50-55 minutes, until the top is golden brown and the cream is bubbling around the edges. The potatoes should be tender when pierced with a knife.
13. Remove from oven and let rest for 5 minutes before serving to allow the cream to set and the dish to become easier to portion.