Preparation

Make the dough

• Combine the flour, sugar, and salt in a stand mixer fitted with a paddle attachment (or if mixing by hand, in a medium bowl). Add the butter and mix, on low, until the mixture looks sandy and there are no pieces of butter larger than a pea, about 2 minutes. The flour should hold together when you clump it with your fingers. Scrape down the sides and bottom with a spatula, if needed. (If mixing by hand, mix with a pastry cutter or two forks until the butter is mixed into the flour as above).
• In a small bowl, mix the egg yolk and milk. With the mixer on low, add them to the flour mixture and mix until the dough just comes together, about 15 seconds; the dough will be somewhat soft. (If mixing by hand, add the yolk mixture to the flour and mix gently with a fork until the liquid is well distributed. The dough will still look crumbly and dry. Dump the dough onto a clean counter and work it with the heel of your hand, pushing and smearing it away from you and gathering it up with a bench scraper and repeating until the dough comes together and is pliable).
• Turn the dough out onto a sheet of plastic wrap, press it into a flat disk, wrap it in the plastic, and let it rest in the refrigerator for 15 to 20 minutes before rolling it out.
• Position a rack in the center of the oven and heat the oven to 350°. Line a heavy-duty rimmed baking sheet with parchment.
• Remove the dough from the refrigerator; if the dough is very firm, let it sit at room temperature until it’s pliable enough to roll, 10 to 15 minutes. On a floured surface, roll the dough into a 13- to 14-inch round. It’s all right if the edges are a little ragged. Transfer the dough round to the baking sheet and put it in the refrigerator while you prepare the filling.

Make the filling

• In a large bowl, toss the raspberries, blueberries, and blackberries with the sugar. Taste the fruit; if it’s more tart than you like, add up to 2 Tbs. more sugar. Toss in the lemon zest, flour and salt until evenly mixed.

Fill and bake the tart

• Remove the dough from the refrigerator and let it sit at room temperature for about 5 minutes to keep it from cracking when you assemble the tart. Heap the fruit in the center of the dough round, leaving a 2-inch border. Fold the edges of the dough over the edges of the fruit to create a rim. Work your way all around, pleating the dough as you go.
• Brush the dough evenly with the egg and sprinkle the sanding sugar directly on the dough and fruit.
• Bake until the dough is completely golden brown, 40 to 55 minutes. (Some of the juices will escape from the tart.) Transfer the baking sheet to a rack to cool.
• When cool enough to handle, use a spatula to transfer the tart to a serving plate or cutting board. Slice it and serve it warm or at room temperature.

Make Ahead Tips

The dough can be made (but not rolled out) up to four days before baking. Keep the disk of dough refrigerated, tightly wrapped in plastic.

The tart may be baked up to six hours ahead of serving.