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1
Preheat oven to 400F.
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2
Heat butter and oil in a 12-inch heavy skillet over moderate heat until foam subsides, then cook onion, stirring occasionally, until golden brown, about 25 minutes.
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3
Remove from heat and stir in thyme, salt, and pepper, then cool.
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4
Toss cheese with onion and spread in bottom of a 9 1/2-inch deep-dish glass pie plate.
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5
Blend flour, baking powder and soda, and salt in a food processor.
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6
Add cheese and pulse 3 or 4 times to combine.
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7
Add butter and pulse until mixture resembles coarse meal.
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8
Whisk together buttermilk and dry mustard in a small bowl and add to processor, then pulse until liquid is incorporated and dough begins to clump (do not let it form a ball).
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9
Turn dough out onto a lightly floured surface and fold it over on itself several times to lightly knead.
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10
Form dough into a ball and let stand, covered with plastic wrap, at room temperature 10 minutes.
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11
Roll out dough between 2 sheets of plastic wrap into a 10-inch round.
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12
Remove top sheet of plastic wrap and flip dough over onto onion.
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13
Peel off remaining plastic and tuck dough inside rim of pie plate.
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14
Bake pie in middle of oven until crust is golden brown and firm to the touch, 25 to 30 minutes.
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15
Cool pie on a rack 2 minutes, then invert a platter over pie and flip pie onto platter.
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16
Serve warm.
