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1
Combine the flour and egg in a bowl with a pinch of salt; quickly mix in the butter with your hands, then add cold water by the tablespoon until you can gather the dough into a ball.
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2
If time allows, refrigerate for 30 minutes or longer (you can leave it in the refrigerator, well wrapped, for up to a day or freeze for up to a month).
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3
Preheat the oven to 450F.Use some butter or olive oil to lightly grease a 10- to 12-inch round pizza pan.
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4
Roll out the dough on a lightly floured surface or simply press it into the pan, right up to the edges.
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5
Bake for about 10 minutes or until it is just beginning to color.
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6
Remove and set on a rack.
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7
(You can proceed immediately or wait an hour or two before finishing the tart.)
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8
Set the oven at 400F.
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9
Spread the crust with mustard.
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10
Core the tomatoes; if they are very juicy, cut in half through the equator and squeeze out some of the juice, shaking out most of the seeds as well.
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11
Cut into about 1/4-inch-thick slices and place in one layer on the crust.
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12
Sprinkle with salt, pepper, and thyme, then drizzle with olive oil.
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13
Bake the tart for about 30 minutes, until the tomatoes are shriveled and hot and the crust is browned.
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14
Serve hot or warm; this does not keep very well.
