1. Preheat oven to 400 degrees. In a food processor, pulse hazelnuts, sugar, and flour until finely ground. In a medium bowl, using an electric mixer, beat butter until light and fluffy, about 1 minute. Beat in egg and vanilla, then hazelnut mixture.

2. Unfold pastry onto a parchment-lined rimmed baking sheet. With an offset spatula, spread hazelnut mixture on pastry, leaving a 1/2-inch border. Shingle nectarines over hazelnut mixture. Bake until pastry is golden and puffed and fruit is tender (if bubbles form in pastry, prick with a knife to deflate), 35 minutes. Brush fruit with preserves; let tart cool 20 minutes before serving.