Step 1

On a lightly floured surface, roll out the dough and press into two six-inch tart pans, trimming off excess. Refrigerate until firm, about 30 minutes.

Step 2

Preheat oven to 375°. Line the dough with parchment paper and top with pie weights or dried beans. Bake for 20 minutes. Remove the parchment and weights. Continue baking until the crust is golden brown and crisp, about 10 minutes more.

Step 3

Transfer tart pans to a wire rack and let cool slightly. Reduce the oven temperature to 350°.

Step 4

Meanwhile, make the filling: In a skillet 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 heat.

Step 5

In a small bowl, stir together the crème fraîche and mustard and spread evenly over the crust. Sprinkle with half of the cheese and the thyme. Arrange the apple slices and onions on top. Sprinkle with the remaining cheese and season with salt and pepper.

Step 6

Bake on the top oven rack until the pastry is golden and the cheese is melted, about 15 minutes. Let cool for 10 minutes, then garnish with thyme, cut into slices and serve.