Make the dough. In a bowl, mix flour, salt, and sugar. Cut in the cold butter with a pastry cutter or your fingertips until pea-sized bits form.
Stir in vinegar, then add ice water a little at a time until the dough just holds when pinched. Bring it together, flatten into a disk, wrap, and chill for at least 1 hour.
Prep the apples. Peel and core the Honeycrisps. Slice thinly, about 1/8 inch.
Toss with lemon juice, zest, sugars, cinnamon, nutmeg, vanilla, cornstarch, and a pinch of salt. Let sit 10 minutes to draw out some juices.
Heat the oven. Set to 400°F (200°C). Line a baking sheet with parchment paper.
Roll the dough. On a lightly floured surface, roll the chilled dough into a 12-inch circle, about 1/8 inch thick.
Transfer to the lined baking sheet.
Arrange the apples. Spoon the apples into the center, leaving a 2-inch border. Fan the slices slightly for a tidy look. Dot the top with small bits of butter.
Fold and seal. Fold the dough edges over the apples, pleating as you go.
Press lightly to help the folds stick. Brush the crust with egg wash and sprinkle with coarse sugar.
Bake. Bake 35–45 minutes, until the crust is golden and the apples are tender and bubbling. If the edges brown too fast, tent loosely with foil for the last 10 minutes.
Cool briefly. Let the galette rest 15–20 minutes.
This helps the juices thicken so slices hold their shape.
Serve. Slice and serve warm. Best with vanilla ice cream or lightly sweetened whipped cream.