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1
Working over a medium bowl, grate the butter on the large holes of a box grater; freeze.
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2
In another bowl, combine the flour, sugar and salt.
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3
Using a pastry blender or 2 knives, cut in the butter until the mixture resembles coarse meal.
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4
Add the cream and mix with a fork until a dough forms.
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5
Turn the dough out onto a lightly floured work surface and pat into a disk.
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6
Wrap in plastic and refrigerate until chilled, at least 1 hour.
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7
In a large skillet, cook the bacon over moderate heat, turning, until crisp, about 6 minutes.
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8
Drain the bacon on paper towels and cut into 1-inch pieces.
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9
Pour off all but 3 tablespoons of the fat from the skillet.
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10
Add the onions to the skillet and cook over low heat, stirring occasionally, until caramelized and very tender, about 1 hour.
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11
Season with salt and pepper and let cool.
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12
Preheat the oven to 425.
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13
On a lightly floured surface, roll out the dough to a 15-inch round.
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14
Fold the dough in half and transfer it to an 11-inch fluted tart pan with a removable bottom.
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15
Unfold the dough and press it into the pan.
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16
Trim the overhang.
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17
Freeze the tart shell until firm, about 10 minutes.
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18
Line the tart shell with foil and fill with pie weights or dried beans.
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19
Bake for about 30 minutes, or until the pastry starts to dry.
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20
Remove the foil and weights and bake for about 3 minutes longer, or until golden brown.
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21
Transfer the tart shell to a rack to cool.
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22
Turn the oven down to 375.
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23
In a bowl, whisk the eggs with the cream.
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24
Stir in the bacon, onions, Gruyere, Parmesan, rosemary, cayenne and nutmeg.
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25
Season with 1/2 teaspoon salt and 1/4 teaspoon pepper.
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26
Pour the filling into the tart shell and bake for 25 minutes, or until the custard is set.
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27
Transfer to a rack to cool slightly.
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28
Unmold and serve.
