Duchess Potato Casserole

Ingredients

• 3 ½ pounds Yukon Gold potatoes, peeled and sliced 1/2 inch thick (NOT CUBED)
• ⅔ cup half-and-half
• 1 large egg, separated, plus 2 large yolks
• Salt and pepper
• Pinch nutmeg
• 10 tablespoons unsalted butter, melted

Directions

1. Adjust oven rack to middle position and heat oven to 450 degrees.
2. Grease (with butter) 13 by 9-inch baking dish.
3. Place potatoes in large saucepan and add cold water to cover by 1 inch.
4. Bring to simmer over medium-high heat. Adjust heat to maintain gentle simmer and cook until paring knife can be slipped into and out of centers of potatoes with no resistance, 18 to 22 minutes.
5. Drain potatoes.
6. While potatoes cook, combine half-and-half, 3 egg yolks, 1 3/4 teaspoons salt, 1/2 teaspoon pepper, and nutmeg in bowl. Set aside.
7. Place now-empty saucepan over low heat; set ricer or food mill over saucepan. Working in batches, transfer potatoes to hopper and process.
8. Using rubber spatula, stir in 8 tablespoons melted butter until incorporated.
9. Stir in reserved half-and-half mixture until combined.
10. Transfer potatoes to prepared dish and smooth into even layer.
11. Combine egg white, remaining 2 tablespoons melted butter, and pinch salt in bowl and beat with fork until combined.
12. Pour egg white mixture over potatoes, tilting dish so mixture evenly covers surface.
13. Using flat side of paring knife, make series of 1/2-inch-deep, 1/4-inch-wide parallel grooves across surface of casserole. Make second series of parallel grooves across surface, at angle to first series, to create crosshatch pattern.
14. Bake casserole until golden brown, 25 to 30 minutes, rotating dish halfway through baking. Let cool for 20 minutes. Serve. 

Notes
- To make this dish easier to pull off, skip the piping and baked the mashed potatoes in a baking dish. To give the casserole a crisp, golden brown, attractive finish, coat the top with a mixture of butter and egg whites and then scored the surface with a paring knife before baking.

- TO MAKE AHEAD: At end of step 3, wrap dish in plastic wrap and refrigerate for up to 24 hours. To serve, top and score casserole as directed in step 4 and bake in 375-degree oven for 45 to 50 minutes.

Source
- https://www.americastestkitchen.com/recipes/10219-duchess-potato-casserole

- https://www.youtube.com/watch?v=dgqYb9LcUSk