In a food processor, pulse flour, sugar, salt, thyme, and frozen butter until mixture resembles coarse crumbs. prep

Add egg yolks and ice-cold water, pulsing until dough forms small, pea-sized clumps. mix

Transfer dough to a lightly floured surface, shape into a disk, wrap in plastic wrap, and chill for 30–45 minutes. prep

Meanwhile, heat butter in a skillet over medium heat. Add shallots and ½ teaspoon salt; sauté for 5 minutes until soft. prep

Slice potatoes very thin using a mandolin, rinse in cold water, and pat dry with a towel. prep

In a clean bowl, toss potatoes with olive oil, 1 teaspoon salt, sautéed shallots, thyme, Parmesan, and mustard until evenly coated. mix

Remove dough from fridge and roll on parchment paper into a 14-inch circle. Place parchment on a baking sheet. prep

Spread Boursin cheese in the center of the dough. Add potato mixture, leaving a 2-inch border. Fold edges over filling, overlapping as needed. assemble