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1
To make the dough, stir the flours together.
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2
Add the butter and rub it with your fingertips until it resembles coarse bread crumbs.
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3
Stir in enough of the water until the dough holds together.
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4
Shape into a thick disk, wrap in plastic wrap, and refrigerate for 30 minutes.
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5
On a lightly floured surface, roll the dough into a 1/8 in (3mm) round circle and use to line a 9in (23cm) tart pan with a removable bottom.
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6
Prick it lightly all over with a fork.
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7
Chill for 30 minutes.
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8
Preheat the oven to 400F (200C).
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9
Line the dough with parchment paper and fill with baking beans.
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10
Bake for 10 minutes.
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11
Remove the paper and beans and bake for 10 minutes more, until the crust is golden and crisp.
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12
Remove from the oven.
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13
Reduce the oven temperature to 375F (190C).
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14
Meanwhile, to make the filling, heat the butter and oil in a large frying pan.
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15
Add the onion and cook over low heat, stirring frequently for 15 minutes, until tender.
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16
Stir in the sugar and nutmeg and cook for about 3 minutes more, until golden.
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17
Transfer to a plate and cool.
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18
Stir half the Gruyere into the cooled onion mixture, then spread in the pastry shell.
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19
Sprinkle with the remaining cheese.
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20
Whisk the half-and-half, eggs, and mustard together and season with salt and pepper.
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21
Pour into the shell.
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22
Bake for 30 minutes, until lightly set.
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23
Let cool on a wire rack for 10 minutes.
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24
Remove the sides of the pan and serve warm or cooled.
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25
Variation:
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26
Roquefort and Walnut Tart: Add 1/2 cup toasted and very finely chopped walnuts to the flour in the pastry dough.
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27
Substitute 2 leeks, white and pale green parts, finely sliced, for the onion, and Roquefort for the Gruyere.
