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1
On a lightly floured work surface, roll out the dough to 1/4 inch thick.
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2
Press the dough onto the bottom and up the sides of an 8 x 11-inch rectangular tart pan with a removable bottom; trim the dough flush with the top edge of the pan.
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3
Prick the bottom all over with a fork.
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4
Transfer to a large rimmed baking sheet.
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5
Freeze until firm, about 30 minutes.
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6
Preheat the oven to 400F.
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7
Line the tart shell with parchment paper, and fill with pie weights or dried beans.
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8
Bake until the dough starts to feel firm on the edges, about 20 minutes.
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9
Remove the parchment and weights; continue baking until the crust is pale golden brown, about 10 minutes.
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10
Let cool completely on a wire rack.
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11
Leave the oven on.
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12
Heat the oil in a large, heavy-bottomed skillet over low heat until hot but not smoking.
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13
Cook the onions, stirring frequently, until golden brown, about 30 minutes.
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14
Let cool.
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15
Whisk the eggs, milk, cream, and salt in a medium bowl; season with pepper.
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16
Crumble in the cheese, and stir in the caramelized onions.
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17
Pour the mixture into the tart shell; bake until puffed and pale golden brown, 30 to 35 minutes.
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18
Let cool at least 30 minutes before serving.
