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1
Place flour and salt into a food processor and pulse. Add cold butter and pulse until butter is in oatmeal-size flakes. Don't wash the food processor bowl; you will be using it to grate the cheese. Place flour/butter mixture into a mixing bowl and stir in the mustard water with a fork. Use only enough liquid to allow pastry to clump onto the fork. Using your hands, gather the dough into a ball and place on a sheet of plastic wrap about 18-inches long. Fold wrap to loosely enclose the dough, then press down to form a disk about 1-inch thick. Refrigerate about an hour or place in freezer about 5 minutes to firm up. While dough is chilling, prepare the filling.
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2
Preheat the oven to 400u00b0 F. Slice the tomatoes about 1/4-inch thick and and place in a pie dish along with basil leaves, garlic, olive oil, and a tiny bit of salt.
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3
Take the Gruyere and grate in the aforementioned food processor.
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4
When dough is chilled, unwrap it and cover with another sheet of plastic wrap. Start rolling it out. It will crumble. Roll it out about 1/2-inch thick. then fold in three. Turn it so you are rolling in the vertical direction. Turn and roll twice and by then the dough should be pliant and the butter should be well distributed. Now make the final roll, placing it, still on the plastic wrap on the bottom part of a 9-inch tart pan with a lightly-floured removable bottom. When the dough is almost the size of the pan bottom, reverse it onto the pan bottom and continue rolling until you have a 1 1/2-inch border around the pan. You should be able to see the dark pan through the dough. Lift the bottom part and fit into the quiche ring. Push the dough up the sides of the ring to about 1/4-inch above it. Refrigerate the dough shell.
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5
Take the pastry shell and spread 1 tablespoon Dijon mustard on the bottom. Add the grated cheese and then overlap the tomato slices with the basil and garlic, leaving the juice and seeds in the plate. Place finished pie onto the bottom shelf of the oven and bake for about 10 minutes, then place on middle shelf and bake an additional 25 minutes. The cheese should be bubbling, the tomatoes just beginning to take on a little brown color and the crust should be golden brown.
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6
Place the quiche pan on top of a bowl a few inches less wide than the pan bottom. The pan bottom will remain on the bowl and he ring will slip down. You can use a sharp knife to make sure the crust sides separate. Then serve after allowing it to cool a bit.
