Spinach, Feta & Root Vegetable Crostata

To prepare crust: Combine all-purpose flour, whole-wheat flour, walnuts and ½ teaspoon salt in a food processor and pulse until the walnuts are coarsely chopped. Add butter and pulse until it is the size of large peas. Add ¼ cup water and pulse until evenly moistened. Add another tablespoon or two of water, if necessary, so that you can squeeze a ball of dough and it doesn't crumble apart. Turn the dough out onto a clean work surface, gather into a ball and wrap in plastic wrap. Refrigerate until ready to use.

To prepare filling: Position racks in upper and lower thirds of oven; preheat to 425°F.

Toss kohlrabi (or turnip) and yellow beets with 1 tablespoon oil and 1/4 teaspoon each salt and pepper in a large bowl. Arrange on one half of a large rimmed baking sheet. Add red beets to the bowl and toss with 1/2 tablespoon oil and 1/8 teaspoon each salt and pepper. Arrange on the other side of the baking sheet. Roast the vegetables on the lower rack until tender and lightly browned, flipping the vegetables halfway through, about 40 minutes.

Meanwhile, heat the remaining 1/2 tablespoon oil in a medium skillet over medium-high heat. Add onion and cook, stirring often, until tender, about 5 minutes. Add garlic and cook for 30 seconds more. Remove from heat. Squeeze spinach dry and add to the onion mixture; stir and let cool.

Process feta, lemon zest, lemon juice, thyme and the remaining 1/8 teaspoon pepper in a food processor until combined. Stir into the cooled spinach mixture.

To prepare crostata: Line a baking sheet with parchment paper.

Roll the dough on a lightly floured surface into a 13-inch circle. Transfer the dough to the prepared baking sheet. Spread the feta mixture over the dough, leaving a 1½-inch border. Arrange the roasted vegetables on top. Fold the edges of the dough toward the center, overlapping as necessary. Brush the exposed crust with some of the egg wash.

Bake the crostata on the lower rack for 20 minutes. Move the pan to the upper rack and bake until the crust is browned and the filling has set, 15 to 20 minutes more. Let cool for 5 to 10 minutes before cutting into wedges. Serve warm or at room temperature.