Preheat the oven to 450°. In a large skillet, heat 2 tablespoons of the oil. Add the onions and cook over moderately high heat, stirring, until softened and lightly browned, about 5 minutes. Transfer to paper towels and blot dry.
Cook the bacon in the skillet over moderate heat until crisp, about 8 minutes. Drain on paper towels, then crumble into small pieces.
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 onions on the pastry, leaving a 1-inch border. Season with salt and pepper and sprinkle with the cheese and bacon. Arrange the potato slices in a single layer on top and season with salt and pepper. Garnish with the rosemary and drizzle on the remaining 2 tablespoons of oil. Fold the edge of the pastry over onto the topping and brush the rim with the egg wash.
Bake the tart for about 20 minutes, or until the pastry is golden. Let cool slightly before serving.
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