To make the crust: Whisk the flour and salt together in a large bowl. Cut the butter into 1/2" cubes. Combine the apple cider vinegar and 1/4 cup (57g) of the ice water., Working quickly, add the butter to the flour and toss to coat. Then use your fingers or the palms of your hands to press each cube of butter into a flat sheet. Keep tossing the butter in the flour as you go to ensure that each butter piece is coated with flour. The idea is to create flat, thin shards of butter that range from about the size of a dime to about the size of a quarter., Sprinkle about 3 tablespoons of the cold vinegar/water mixture over the flour mixture. Use a gentle hand or wooden spoon to stir the water into the flour until just combined. If the dough seems dry, add more cold water a couple of teaspoons at a time. You've added enough water when you can pick up a handful of the dough and easily squeeze it together without it falling apart., Press the dough together, and shape into a disk. Wrap and chill the dough for at least 2 hours before using, but preferably overnight., Position a rack in the center of the oven and preheat the oven to 400°F., On a lightly floured piece of parchment paper, roll the pastry disk into an oval about 15" x 6" and just under 1/4" thick. Use a paring knife or pastry cutter to trim away any rough edges and move the parchment sheet and crust to a baking sheet. Prick the crust all over with a fork to prevent it from puffing up in the oven., Brush the surface of the crust from edge to edge with the egg wash. Bake the crust until it's deep golden brown, 25 to 30 minutes. Check it halfway through baking and if any bubbles have appeared, use a spatula to press them flat. Cool the crust completely on the pan., To make the filling: While the crust is cooling, combine the mascarpone and 2 tablespoons of the sugar. Hull the strawberries and cut them into 1/4" slices., Move the cooled crust to a serving platter or board and spread the mascarpone over the top in an even layer. Dot with the jam, then arrange the sliced strawberries in a single overlapping later in a decorative pattern., Sprinkle the tart with the remaining tablespoon of sugar (omit this final sprinkling if your strawberries are particularly sweet), slice, and serve immediately.