Step 1

Bake the whole sweet potatoes: Arrange a rack in the middle of the oven and heat to 375°F. Place the sweet potatoes directly on the oven rack (place a piece of aluminum foil onto the rack below if you're worried about drips). Bake until tender, about 1 hour.

Step 2

Cool and store the potatoes: Cool the potatoes for about 30 minutes, or until you can comfortably handle them. Scoop the tender orange flesh out of the sweet potato peels and store in the refrigerator for up to 3 days before proceeding.

Step 3

Prepare the baking dish: Generously coat a 9x13-inch or 3-quart baking dish (glass or ceramic) with the 1 tablespoon room-temperature butter; set aside.

Step 4

Mash the sweet potatoes: If you baked the potatoes in advance, remove them from the fridge about 30 minutes before making the casserole, as this will make them easier to mash and blend. Place the sweet potato flesh, 4 tablespoons melted butter, brown sugar, salt, thyme, cinnamon, and nutmeg in a large bowl. Mash until mostly smooth and well-combined. Some lumps are welcome.

Step 5

Add the egg yolks: Mix in the egg yolks with a wooden spoon until well-incorporated.

Step 6

Bake the casserole: Transfer the sweet potato mixture into the prepared baking dish and spread into an even layer. Bake for 30 minutes. Make the meringue topping while the casseroles bakes.

Step 7

Make the meringue: In a stand mixer fitted with the whisk attachment or with an electric hand mixer, whisk the egg whites until foamy, about 1 minute. Gradually add the sugar while mixing, and whip the meringue to stiff peaks.

Step 8

Top and brown: Carefully spread or pipe the meringue onto the sweet potato mixture, covering as much of the sweet potato as possible. Bake until the meringue is browned, about 10 to 15 minutes more.