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1
Butter a 10-inch tart pan with removable bottom; set aside.
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2
Make the crust: In a medium bowl, combine flour and 1/2 teaspoon salt.
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3
Add 8 tablespoons (1 stick) chilled butter, and combine with fingers or a pastry blender until it has the texture of coarse meal.
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4
Add 1/2 cup ice water, and stir just until a dough forms.
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5
Form into a disk, and wrap in plastic wrap.
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6
Chill for at least 15 minutes.
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7
Preheat oven to 375.
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8
On a lightly floured surface, roll out dough to 1/8-inch-thick round.
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9
Fit dough into prepared tart pan; trim excess.
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10
Line with a parchment paper round, and fill with pie weights.
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11
Chill for 15 minutes.
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12
Transfer to oven, and bake for 15 minutes.
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13
Remove pie weights and parchment paper.
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14
Bake for 5 minutes more.
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15
Remove from oven, and set aside.
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16
In a large skillet, heat remaining 4 tablespoons butter.
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17
Add onions, and cook, stirring frequently, until slightly browned and softened, about 15 minutes.
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18
Remove from heat.
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19
In a small bowl, combine beaten egg, cream, remaining 1 teaspoon salt, pepper, and nutmeg.
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20
Stir in onions.
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21
Sprinkle baked tart shell with cheese.
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22
Spread onion mixture evenly over cheese.
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23
Bake until set, 35 to 40 minutes.
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24
Serve immediately.
