Preheat oven to 450°. On a lightly floured sheet of parchment paper, roll out the tart pastry (see recipe) to a 13 inch round. Transfer the pastry and parchment paper to a baking sheet.
Saute the onions in oil just until softened. Spread onions on pastry, leaving a 1 inch border. Sprinkle olives over onions. Fold the edge of the pastry over onto the onions and brush the rim with egg wash. Bake the tart for 10 minutes. Spread the shredded chicken and grated cheese over the onion/olive mixture. Bake for 10 minutes or just until pastry is golden and cheese is melted. Cool slightly, garnish with parsley and serve warm. 4 servings.
In a food processor, combine the flour, sugar and salt. Add the butter and pulse until the mixture resembles coarse meal. Add 2 1/2 TBS water and pulse just until the pastry comes together; add more water if necessary. On a lightly floured surface, shape the pastry into a disk, wrap in plastic and refrigerate until chilled, about 30 minutes. Can be made ahead and refrigerated up to 2 days or frozen for one month.