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1
Roll out dough on a lightly floured surface with a lightly floured rolling pin into an 11-inch round and fit into tart pan, trimming excess dough.
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2
Chill until firm, about 30 minutes.
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3
Put oven rack in middle position and preheat oven to 375F.
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4
Lightly prick bottom of shell all over with a fork, then line with foil and fill with pie weights.
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5
Bake until side is set and edge is pale golden, 18 to 20 minutes.
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6
Carefully remove foil and weights and bake shell until bottom is golden, 10 to 15 minutes more.
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7
Cool completely in pan on a rack, about 15 minutes.
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8
Heat butter and oil in a 12-inch heavy skillet over moderately high heat until foam subsides, then saute mushrooms, shallot, thyme, 1/2 teaspoon salt, and 1/4 teaspoon pepper, stirring frequently, until mushrooms are tender and any liquid given off is evaporated, 8 to 10 minutes.
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9
Transfer to a bowl and cool to room temperature.
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10
Whisk together creme fraiche, heavy cream, whole egg, yolk, and remaining 1/4 teaspoon salt and 1/8 teaspoon pepper in a medium bowl until combined.
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11
Reduce oven temperature to 325F.
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12
Scatter mushrooms evenly in tart shell and pour custard over them.
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13
Bake tart in pan on a baking sheet until custard is just set and slightly puffed, 35 to 45 minutes.
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14
Cool tart in pan on rack at least 20 minutes, then remove side of pan.
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15
Serve tart warm or at room temperature.
