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. 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).
Preheat the oven to 450°F.Use some butter or olive oil to lightly grease a 10- to 12-inch round pizza pan. Roll out the dough on a lightly floured surface or simply press it into the pan, right up to the edges. Bake for about 10 minutes or until it is just beginning to color. Remove and set on a rack. (You can proceed immediately or wait an hour or two before finishing the tart.)
Set the oven at 400°F. Spread the crust with mustard. 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. Cut into about 1/4-inch-thick slices and place in one layer on the crust. Sprinkle with salt, pepper, and thyme, then drizzle with olive oil. Bake the tart for about 30 minutes, until the tomatoes are shriveled and hot and the crust is browned. Serve hot or warm; this does not keep very well.
