How to Make It
Preheat the oven to 450°. Line a large baking sheet with parchment paper. On a lightly floured surface, roll out the dough to a 12-by-16-inch rectangle; transfer to the baking sheet. Fold the dough edge over itself to form a 1/2-inch border all around and pinch the corners together. Refrigerate until firm.
Spread the crème fraîche and pecorino over the dough; top with overlapping slices of the potato and apple in rows. Brush with butter, sprinkle with thyme and season with salt and pepper. Brush the edges with the egg wash.
Bake the galette in the bottom third of the oven for 25 minutes, or until the edges are slightly puffed. Cut into squares; serve warm or at room temperature.