To make the dough: Place the flour, and salt in the bowl of the food processor fitted with the metal blade and pulse to combine. Add the butter and toss it around to coat the cubes with the flour. Pulse the mixture just until the butter is the size of peas. With the motor on, pulse the ice water in through the feed tube, stopping just before the dough starts to form a ball. Turn the dough onto a floured board and flatten into a disk. Wrap in plastic wrap and chill for 1 hour.

While the dough is chilling, heat the butter in a large saucepan over medium heat and add the onions, parsley and a pinch or two of salt and pepper. Sauté until the onions are tender and beginning to brown, stirring often. Remove from the heat and set aside to cool a bit.

Place the tomatoes on a baking sheet, sprinkle with a little salt and pepper and drizzle them with a little olive oil. Set aside.

Preheat oven to 450ºF. Turn a rimmed sheet pan upside down and place a piece of parchment paper on it. Flour the board again and roll the dough out into an 11 or 12 inch circle. Spread the onions across the dough, leaving a 1 ½-inch border all the way around. Sprinkle the cheese evenly across the top of the onions and arrange the tomato slices, overlapping them in concentric circles till you’ve used up all the tomatoes. Fold the edges of the dough up and over the tomatoes, crimping them down so they lie flat. Brush the dough with the egg wash.

Bake for about 30 minutes, until the pastry is golden brown. Let sit on the pan for about 5-7 minutes to set up and then sprinkle with the parsley, cut into slices and serve.

You can definitely make this early in the day and serve later. Leave the tart on the inverted pan and just pop into the oven at 350ºF for about 10-12 minutes before you are ready to serve.