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1
In a food processor, pulse the flour with 1/2 teaspoon of the salt.
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2
Add the diced butter and pulse until it's the size of small peas.
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3
Sprinkle in the cold water and pulse again until the tart dough begins to come together.
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4
Turn the dough out onto a sheet of plastic wrap and pat it into a disk.
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5
Wrap the dough and refrigerate until it is firm, at least 1 hour.
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6
Preheat the oven to 400.
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7
Meanwhile, in a large skillet, heat the oil.
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8
Add the onions and cook over moderate heat, stirring occasionally, until soft and lightly golden, about 25 minutes.
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9
Reduce the heat to low and continue to cook, stirring occasionally, until the onions are golden, about 15 minutes longer.
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10
Stir in the thyme and season with salt and pepper.
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11
Remove from the heat and let the onions cool to room temperature.
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12
On a lightly floured work surface, roll out the dough to a 12-inch round.
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13
Wrap the dough around the rolling pin and transfer it to a 10-inch fluted tart pan with a removable bottom.
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14
Press the dough into the pan and trim off any excess.
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15
Use the scraps to patch any cracks.
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16
Prick holes all over the bottom of the crust with a fork.
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17
Bake until the crust is lightly browned, about 20 minutes.
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18
Spread the onions in the tart shell and sprinkle with the blue cheese.
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19
Bake until the cheese is melted and the onions are warm, about 5 minutes.
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20
Cut the tart into wedges and serve.
