Potato, Onion, Gruyere Galette

Directions

1. Preheat the oven to 375°,F and position an oven rack in the lower third. Line a baking sheet with parchment paper or a silicone mat. Roll the pastry dough into a 13-inch circle and transfer it to the baking sheet. Chill for 1 hour.
2. Heat the 1½, tablespoons olive oil in the medium sauté, pan over medium-high heat. Add the onion and cook, stirring occasionally, until soft and lightly colored, 8 to 10 minutes. Stir in the thyme, ¼, teaspoon salt, and 5 grinds pepper and blend well. Transfer to a plate and set aside to cool.
3. Combine the cooled onion mixture, cheese, and potatoes in the large bowl. Mound the filling in the center of the chilled tart shell, leaving a 1½,-inch border at the edge. Fold that border up around the filling, pleating it to make a pretty, circular enclosure and leaving the center open.
4. Drizzle the filling with the remaining 1 tablespoon olive oil and sprinkle lightly with salt and 3 grinds of pepper. Lightly brush the pleated dough with the beaten egg to give it shine and help it brown in the oven.
5. Bake the galette for 45 to 50 minutes, until the pastry is golden brown and the potatoes are soft when tested with a paring knife or skewer. Use the metal spatula to lift the edge of the galette slightly and check underneath. The bottom crust should be a beautiful brown color. Transfer to a rack to cool for 5 to 10 minutes.