Makes one 10-inch tart
For the crust
1 1/4 cups all-purpose flour
2 tablespoons cornmeal
1/2 teaspoon salt
1/2 cup (1 stick) cold butter, cubed
2-3 tablespoons ice water
For the tart filling
2 1/2 tablespoons extra-virgin olive oil, divided
1 large sweet yellow onion, peeled and thinly sliced
Salt and coarsely ground black pepper
1 small butternut squash (1 1/2 pounds), trimmed, peeled, halved lengthwise, and cored
2 tablespoons minced fresh sage
1 1/2 cups grated Gruyere cheese
Prepare the crust dough: In the work bowl of a food processor, combine the flour, cornmeal, and salt; pulse to combine. Scatter the butter over the flour mixture, and pulse until the mixture resembles coarse meal, 10-12 pulses. With the machine running, add 2 tablespoons of the ice water through the feed tube in a slow stream, mixing until the dough just begins to hold together, adding more water by the teaspoon if necessary. Do not overprocess, or the dough will become tough.
Turn the dough out onto a work surface and gather it into a ball. Flatten the ball into a disk, and wrap it in plastic wrap. Chill the dough for at least 1 hour before proceeding.
Meanwhile, start on the filling: Heat one tablespoon of the olive oil in a large skillet set over medium-low heat. Add the slice onion to the skillet, and sprinkle it with salt and pepper. Cook until the onion is deep golden brown, stirring occasionally, 25-35 minutes. Set skillet aside to cool slightly.
Thinly slice each half of the squash crosswise. Toss the squash with the remaining 1 1/2 tablespoons olive oil and the sage in a large bowl. Sprinkle with salt and pepper.
Preheat the oven to 425F degrees.
Remove the crust dough from the refrigerator. On a lightly floured work surface, roll the dough out to a 13-inch round. Carefully transfer the dough to a fluted 10-inch tart pan with a removeable bottom, pressing evenly into the edges. Trim the dough so that it is flush with the edge of the tart pan. Freeze until firm, about 15 minutes.
Spread the caramelized onions over the bottom of the tart pan. Sprinkle the Gruyere over the onions. Arrange the squash slices in concentric circles over the cheese, overlapping as necessary.
Place the tart on a baking sheet and bake for 35-45 minutes, until
the squash starts to caramelize and the crust becomes golden brown.
Allow the tart to rest for at least 10 minutes before cutting, or allow
it to cool completely and serve at room temperature.