Free-Form Apple–Walnut Tart

Dough:

• 2/3 cup all-purpose flour
• 2 tablespoons cornmeal
• 2 teaspoons sugar
• 1/4 teaspoon salt
• 4 tablespoons cold unsalted butter, cut into 1/2-inch pieces
• 2 tablespoons ice water
• 2 tablespoons cold buttermilk

Apple-Walnut Filling:

• 1/4 cup walnut pieces
• 3 tablespoons plus 1/2 teaspoon sugar
• 2 tablespoons all-purpose flour
• 2 Golden Delicious apples, peeled, cored, and very thinly sliced
• 1 tablespoon butter, cut into 1/4-inch pieces

To make the dough, combine the flour, cornmeal, sugar, and salt in the bowl of a food processor. Drop in the butter and pulse until it's the size of small peas. Combine the water and buttermilk. With the machine running, quickly pour the combined liquids into the flour mixture. Turn off processor as soon as the dough comes together, about 5 seconds. Gather together the dough on a floured work surface and form into a disk. Wrap in plastic and refrigerate for 2 hours.

Preheat oven to 400ºF and line a baking sheet with parchment paper. Roll the dough out to a 10-inch circle of about 1/8-inch thickness. Dust both sides with flour, gently fold into quarters, and transfer to the prepared baking sheet. Set in the refrigerator for 15 minutes.

Combine the walnuts, 3 tablespoons of the sugar, and the flour in the bowl of a food processor and pulse until nuts are coarsely ground. Remove the dough from the refrigerator and sprinkle on about half of the nut mixture in an even layer, leaving a 1 1/2-inch border around the edge of the dough. Arrange half of the apple slices on top of the walnut mixture and top with some of the remaining walnut mixture. Arrange the remaining apples and top with the rest of the walnut mixture. Fold the border of the crust over the apples, brush the crust with water, and sprinkle with the remaining sugar. Dot the top of the apples with butter. Place on the bottom rack of the oven and bake until the crust is golden brown and the apples are tender when pierced with a knife, 35 to 40 minutes.