Step 1

To make the crust, in a food processor, combine the butter, flour and salt and pulse until the mixture resembles coarse meal with some large pieces. In a small bowl, whisk together the egg, egg yolk and ice water. While pulsing, drizzle in the egg mixture until a dough forms. Turn the dough out onto a sheet of plastic wrap, shape into a rectangle and wrap well. Refrigerate until firm, at least 1 hour or up to 2 days.

Step 2

On a lightly floured surface, roll out the dough into a 14-by-21-inch (35-by-53-cm) rectangle and refrigerate until firm, about 15 minutes. Coat a nonstick baking sheet with cooking spray. Fit the dough into the prepared baking sheet. Fold the excess dough under and pinch to form a crust that comes 1/2 inch (12 mm) above the rim. Freeze until firm, about 30 minutes.

Step 3

Position 1 rack in the upper third and 1 rack in the lower third of an oven. Preheat to 375°F (190°C).

Step 4

Line the dough with parchment paper, pressing it flush against the surface of the dough and leaving a 2-inch (5-cm) overhang on all sides. Top with pie weights or dried beans. Fold the parchment over the edges of the crust. Bake on the lower oven rack for 25 minutes, rotating the baking sheet halfway through. Remove the parchment and weights. Continue baking until the crust is golden brown and crisp, 15 to 17 minutes more. Transfer the baking sheet to a wire rack and let cool slightly. Reduce the oven temperature to 350°F (180°C).

Step 5

Meanwhile, make the filling: In a large fry pan over medium heat, melt the butter with the olive oil and swirl the pan to combine. Add the onions and cook until lightly browned and starting to caramelize, about 8 minutes per side. Remove from the heat.

Step 6

In a small bowl, stir together the crème fraîche and mustard and spread evenly over the crust. Sprinkle with 1 cup (4 oz./125 g) of the cheese and the thyme. Arrange the apple slices and onions on top. Sprinkle with the remaining 1/2 cup (2 oz./60 g) cheese, and season with salt and pepper.

Step 7

Bake on the top oven rack until the pastry is golden and the cheese is melted, about 25 minutes. Let cool for 10 minutes, then garnish with thyme, cut into slices and serve. Serves 6.

Step 8

Williams-Sonoma Test Kitchen