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Preheat the oven to 450°. On a lightly floured sheet of parchment paper, roll out the Flaky Tart Pastry to a 13-inch round. Transfer the pastry and parchment to a baking sheet.
Spread the sliced onions on the pastry in a single layer, leaving a 1-inch border. Sprinkle the chopped olives all over the onion topping. Fold the edge of the pastry over onto the onions and brush the rim with the egg wash. Bake the tart for 10 minutes.
Spread the shredded chicken on the tart and sprinkle with the cheese. Bake the tart for about 10 minutes longer, or until the pastry is golden and the cheese has melted. Let cool slightly. Just before serving, sprinkle the parsley and chives on top. Serve warm.