MUSHROOM & GRUYÈRE TART WITH HAZELNUT HARICOTS VERTS
All-purpose flour, for dusting 1 sheet frozen puff pastry (half of a 17.3-oz/490-g package), thawed 5 ounces (155 g) Gruyère cheese, shredded 2 leeks, trimmed, halved lengthwise, white and pale parts thinly sliced ¼ lb (125 g) white mushrooms, brushed clean and thinly sliced 4½ tablespoons (70 ml) olive oil Kosher salt and freshly ground pepper 1 large egg beaten with 1 tablespoon water ¾ lb (375 g) haricots verts, trimmed ¼ cup (11/4 oz/40 g) hazelnuts, roughly chopped
1 Preheat the oven to 400ºF (200ºC). Line a sheet pan with parchment paper.
2 On a lightly floured work surface, roll out the puff pastry into a 121/2-by-15-inch (32-by-38-cm) rectangle. Fold into thirds, transfer to the prepared pan, and unfold, positioning the dough so there will be room for the haricots verts on the pan. Fold over about 1 inch (2.5 cm) of each side of the dough to create a border.
3 Sprinkle the dough with the cheese, leaving the borders uncovered. In a bowl, toss together the leeks, mushrooms, and 3 tablespoons of the oil, and season with salt and pepper. Spread the mixture over the cheese. Brush the borders with the egg mixture. Bake for 10 minutes.
4 In a bowl, toss together the haricots verts and the remaining 1½ tablespoons oil, and season with salt and pepper. Place in a single layer on the pan next to the tart. Continue baking until the tart is golden brown and the haricots verts are fork-tender, about 15 minutes longer. During the last 5 minutes of cooking, sprinkle the hazelnuts over the haricots verts.
5 Let the tart cool slightly, then cut into slices and serve the haricots verts on the side.