Onion and Pancetta Tart

To make the crust, put the walnuts in a food processor and pulse until they are very fine and uniform, though not to the point of walnut butter. Add the flour, sugar and salt and pulse a few times to blend. Add the butter and pulse again until the largest piece is the size of a small pea.

With the processor running, drizzle in the water and process until the mixture climbs up the sides of the processor. Remove the top and squeeze a big pinch of the dough to see whether it's still dry and crumbly or holds together and feels moist. If it is dry, pulse in a few more drops of water.

When the dough is the right consistency, dump it on a lightly floured counter and gather it into a ball. Push the dough away from you with the heel of your hand and then, with a dough scraper or thin spatula, scrape it back into a ball. Repeat for a few strokes until the dough starts to come together. Don't overwork it; it's okay if it's still slightly crumbly.

Divide the dough into 2 pieces, one of them using a third of the dough and the other using two-thirds. Shape both pieces into flat disks and wrap them separately in plastic wrap. Freeze the smaller disk for another use. Chill the larger disk in the refrigerator for about 30 minutes; if you chill it longer, leave at room temperature for a few minutes before rolling, to avoid cracking.

Preheat an oven to 400°F (200°C).

On a lightly floured work surface, roll the larger disk of dough into a 13-inch (33-cm) round. Gently roll the dough around your rolling pin, transfer to a 10-inch (25-cm) round tart pan with a removable bottom, and unroll. Gently press the dough down into the corner where the base meets the sides, and press the dough up the sides, leaving some above the rim. Pinch the dough to form a tidy, evenly thick rim.

Line the dough with parchment paper or foil and fill with dried beans or pie weights. Bake until the dough is set and the rim is light brown, about 10 minutes. Reduce the oven temperature to 325°F (165°C). Carefully remove the parchment and beans, return the tart shell to the oven and cook until the base looks dry and is light gold, another 20 minutes. Let the crust cool slightly. (Leave the oven on.)

As the tart shell bakes, make the filling: In a large skillet or Dutch oven over medium-high heat, warm a small glug of olive oil. Add the pancetta and as soon as it sizzles, reduce the heat and cook until the fat has rendered and pancetta is starting to brown, about 7 minutes; don't let it get fully browned and crisp.

Add the onion, season generously with salt and pepper, and cook gently until the onion becomes very soft and fragrant and is a rich golden brown, stirring and scraping the pan frequently, about 30 minutes. Let the onion cool to room temperature.

In a bowl, whisk together the egg yolk and cream. Season lightly with salt and pepper. Fold in the Gruyère and onion. Fill the tart shell with the mixture, smoothing the top.

Bake until the filling is just set, about 20 minutes. Let the tart cool to warm before cutting and serving. Makes one 10-inch (25-cm) tart.

Adapted from Six Seasons: A New Way with Vegetables, by Joshua McFadden with Martha Holmberg (Artisan Books). Copyright © 2017.