Next up is making the tart dough. If you’re used to the typical food processor tart dough method, this is going to be different, but the resulting texture is so much better - flaky perfection. If the instructions are at all confusing you can watch the method here. I learned the technique from Pim years ago and never looked back. Also, you don’t have to wash a food processor = less dishes. To start, mix the flours, 1/4 teaspoon salt, and quinoa in a bowl and turn out onto the counter. Working quickly, cut the butter into four long slabs and arrange in a single layer across the surface of the flour. Hold a bench scraper (or wide metal spatula) in one hand, and use the palm of your other hand to smear the butter into the flour. Use the bench scraper to scrape the smeared flour-butter off the counter and back up into the pile. Use your palm to smear some more, scrape, and repeat. You want lots of feathery flakes throughout the flour. Drizzle the mixture with the ice water and smear, flip, and mix a few more times to bring it all together into a ball. Decide how many tarts you’d like to bake, and cut into that many equal pieces (1, 2, or 4). Shape into balls, press into discs, wrap in plastic and refrigerate for 45 minutes or over night. You can also double wrap and freeze for up to a month or so at this point, thaw before using.
When you’re ready to assemble your tart(s), preheat the oven to 400F with a rack in the center. Roll your dough out into a 1/8-inch thick circle shape on a floured surface, it doesn't have to be perfect. Transfer the rolled out dough to a piece of parchment paper. Arrange a generous amount of the onions in the center of your tart(s) leaving an inch-ish of border. Cover the onions with overlapping sliced tomatoes, then fold the tart dough in toward the filling, over lapping it just a bit. Brush any visible pasty with the beaten egg. Transfer, on the parchment paper, to a baking sheet and bake for 25-30 minutes or until the crust is deeply golden.
Makes one large tomato tart, or two 7-inch tarts, or four smaller individual tarts.