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