Mix together the flour and salt. Cut the cold butter into small cubes and work it into the flour using a pastry cutter or a fork until the mixture looks crumbly.

In another bowl, beat the egg, water, and vinegar together. Combine the wet mixture with the flour mixture, mixing just until the dough starts to come together. If the dough is too dry, add a small amount of water until it clumps together. Wrap the dough in plastic wrap and chill it in the refrigerator for at least 30 minutes.

Peel the apples and slice them into 1/4-inch thick pieces. In a bowl, toss the apple slices with brown sugar, cornstarch, and cinnamon until evenly coated.

On a floured surface, roll out the chilled dough into a rough circle. Don’t worry about making it perfect; just ensure there are no holes. Move the dough circle onto a parchment-lined baking sheet.

Place the apple slices on the dough in a spiral pattern, starting from the center and leaving some space around the edges for folding the dough over to form a crust.

Brush the folded crust edges with the beaten egg and sprinkle the coarse sugar on top.

Bake the galette in a preheated oven at 400°F for about 35 minutes, or until the crust is golden brown. Enjoy your delicious apple galette!