Adapted from Cook’s Illustrated, January/February 2012
Serves 6
Dough
1-3/4 cups (6-1/4 oz.) all-purpose flour
1/2 cup (2-3/4 oz.) whole-wheat flour
1 tablespoon sugar
3/4 teaspoon salt
10 tablespoons unsalted butter, cut into 1/2″ pieces and chilled
7 tablespoons ice water
1 tablespoon white vinegar
Filling
6 oz. baby spinach
1-1/4 lbs. butternut squash, peeled and cut into 1/2″ cubes
5 tablespoons olive oil
1 red onions, sliced thin
1/2 teaspoon minced fresh oregano
3 oz. gruyere cheese, shredded (3/4 cup)
2 tablespoons creme fraiche
1 teaspoon sherry vinegar
salt and pepper
1 large egg, lightly beaten
kosher salt
2 tablespoons minced fresh parsley
For the Dough:
1. Pulse flours, sugar and salt in the food processor until combined, 2-3 pulses. Add butter and pulse until it is in pea-sized pieces, about 10 pulses. Transfer to a medium bowl.
2. Sprinkle water and vinegar over mixture. With rubber spatula, fold mixture until a loose mass forms with some dry flour remaining. Transfer to a sheet of plastic wrap, shape into a thick square and wrap tightly. Refrigerate at least 45 minutes.
3. Transfer dough to a lightly floured work surface and roll into an 11×8″ rectangle. Using a bench scraper, fold in threes like a business letter. Turn and roll again into an 11 x 8″ rectangle and fold into thirds again. Repeat by turning and folding one more time and then fold in half to create a 4″ square. Press top of dough to seal, wrap in plastic wrap and refrigerate for at least 45 minutes or up to two days.
For the Filling:
4. Adjust oven rack to lower-middle position, place pizza stone on rack and heat to 400 degrees.
5. Place spinach and 1/4 cup water in a large microwave-safe bowl. Cover with a large dinner plate and microwave on high 3-4 minutes, or until spinach is reduced by half. Using potholders, carefully removed from microwave and keep covered for 1 minute. Carefully remove plate and transfer spinach to colander. Gently press spinach against sides of colander to release liquid. Transfer to cutting board, chop roughly and return to colander to press again. Add squash to now empty bowl and microwave until tender, about 8 minutes.
6. Meanwhile, heat 1 tablespoon olive oil in a 12″ skillet over medium heat until shimmering. Add onion and oregano, cover, and cook until onions are tender and beginning to brown, 5-7 minutes, stirring frequently. Remove from heat, add onion mixture to squash along with spinach, cheese, creme fraiche and vinegar and stir gently to combine. Season to taste with salt and pepper.
Assemble and Bake:
7. Remove dough from refrigerator and let stand for 15-20 minutes. Roll out on generously floured work surface (or directly onto a sheet of parchment) to a 14″ circle. Transfer to a (parchment-lined) rimmed baking sheet. With a plastic straw or paring knife, cut five small holes into dough — one in the center and four evenly-spaced midway from center to edge of dough. Brush top of dough with 1 teaspoon olive oil.
8. Spread filling evenly over dough leaving a 2″ border around the edge. Drizzle remaining teaspoon olive oil over filling. Carefully grasp one edge of dough and fold up outer 2″ over filling. Repeat around circumference of tart, overlapping dough every 2-3″. Gently pinch pleated dough to secure. Brush dough with egg and sprinkle evenly with kosher salt.
9. Lower oven temperature to 375 and bake until crust is deep golden brown and filling is beginning to brown, 35-45 minutes. Cool tart on baking sheet on wire rack for 10 minutes. Carefully slide tart off parchment onto cutting board. Sprinkle with parsley, cut into wedges and serve.