Step 1

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).

Step 2

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).

Step 3

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.

Step 4

Position a rack in the center of the oven and heat the oven to 350°. Line a heavy-duty rimmed baking sheet with parchment.

Step 5

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.

Step 6

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.

Step 7

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.

Step 8

Brush the dough evenly with the egg and sprinkle the sanding sugar directly on the dough and fruit.

Step 9

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.

Step 10

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.