The key to a flaky crust in this recipe is to keep the dough cold (think apple pie crust). It might seem like overkill, but by constantly returning the dough to a cold temperature you ensure bursts of steam from the butter.
I used a few medium sized heirloom tomatoes to make two ten inch tarts, but there's no reason you couldn't use larger tomatoes and make one large tart (just adjust the baking time accordingly). —Brian Mitchell
In a medium mixing bowl, mix flour and a pinch of salt.
Meanwhile, whisk egg together with ice water.
Remove bowl from freezer. Slowly add egg mixture and mix to incorporate evenly. Form dough into a ball and return to freezer a few minutes.
Meanwhile, slice tomatoes width-wise. Don't slice to thick or it will be difficult to overlap the tomatoes in the tart.
Remove dough from freezer and place on lightly floured surface. With a floured rolling pin, roll dough out to around a 1/8" thickness.
Take a 10" mixing bowl and place it upside down on the rolled out dough. With a paring knife, cut around the perimeter of the bowl. Keeping the bowl in place, pull the excess dough up and use for the next tart.
Line a baking sheet with parchment paper, and place dough rounds on parchment paper. Spread a thin layer of mustard over the top of the dough, leaving a 1" border on the outside.
Place tomatoes on mustard (again, with a 1" border), overlapping in a circle. It helps me to start in the middle and work out. If using different tomato types, alternate them (or come up with a pattern of your choosing!)
With floured fingers, fold sides of tart up. Every 2" or so, pinch and fold a little corner in the dough, working in the same direction for each pinch. When finished, lightly press sides of folds down so they hug the tomatoes and overlap them slightly.
Whisk an egg and a little water together to form an egg wash. Brush wash on sides of the tart dough.
Drizzle tomatoes with olive oil and add pieces of goat cheese (be liberal). Sprinkle Herbes de Provence over top.
Bake in oven for 15-20 minutes or until the crust is golden, and the tomatoes and goat cheese are cooked. If cheese isn't brown or bubbly enough for your likening, set under the broiler for a few minutes.
Let tarts rest a couple of minutes and sprinkle with a little sea salt and freshly cracked pepper to taste.