Potato Galette

Combine the flour, sugar, salt, thyme, and cold butter in a food processor. Pulse until the mixture forms pea-sized crumbs.
Add the egg yolks and ice-cold water to the mixture and pulse until the dough comes together but remains slightly crumbly.
Transfer the dough to a lightly floured surface and shape into a disk. Wrap in plastic and refrigerate for 30 minutes.
While the dough chills, heat butter in a skillet over medium heat. Add shallots and ½ teaspoon salt, sautéing until soft, about 5 minutes.
Rinse potato slices in cold water, then pat dry thoroughly with a kitchen towel.
In a large bowl, toss the potato slices with olive oil, 1 teaspoon salt, cooked shallots, thyme, Parmesan, and mustard until evenly coated.
Remove dough from the fridge and place on parchment paper. Roll into a 14-inch circle, keeping thickness uniform.
Place the Boursin cheese in the center of the dough, then mound the potato filling on top, leaving a 2-inch border.
Fold the edges of the dough over the filling, overlapping slightly and pressing gently to seal.
Brush the exposed crust with the beaten egg yolk using a pastry brush.
Bake at 375°F (190°C) for 35–40 minutes, until the crust is golden and crisp.
Let the galette cool on a wire rack for 5 minutes before slicing.