Spinach, Feta & Root Vegetable Crostata

1. To prepare crust: Combine all-purpose flour, whole-wheat flour, walnuts and ½ teaspoon salt in a food processor; pulse until the nuts are coarsely chopped. Add butter and oil; pulse until lumps the size of large peas form. Add ¼ cup water; pulse until evenly moistened. Add 1 to 2 tablespoons more water, if necessary, so you can squeeze a ball of dough and it doesn't crumble apart. Gather the dough into a ball and wrap in plastic wrap. Refrigerate.

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

3. 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 half of a large rimmed baking sheet. Toss red beets with 1/2 tablespoon oil and 1/8 teaspoon each salt and pepper. Arrange on the other side of the pan. Roast the vegetables on the lower rack until tender and lightly browned, flipping them halfway through, about 40 minutes.

4. 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; cook for 30 seconds more. Remove from heat. Squeeze spinach dry and add to the onion mixture; stir and let cool.

5. 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.

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

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

8. Bake the crostata on the lower rack for 20 minutes. Move 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. Serve warm or at room temperature.

Tips

To make ahead: Refrigerate dough, vegetables and feta mixture (Steps 1-5) separately for up to 1 day. Let the dough stand at room temperature for 15 minutes before rolling.