Frozen puff pastry comes in various sizes, sometimes as one large sheet of pastry in the package, sometimes two smaller pieces. If your package contains two sheets, make two smaller tarts instead of one big one.
Don’t forget to thaw your puff pastry according to package instructions before making this recipe. Pastry that is still frozen or partially-frozen will crumble and crack as you work with it.
1 Preheat the oven to 375°F. Line a baking sheet with parchment paper. (If your package of puff pastry contains two pieces, bake on two separate baking sheets.)
2 Prepare the pastry: Unwrap the pastry, and if needed, unfold. Transfer to a baking sheet. The pastry should already be about 1/8-inch thick, which is what you want; if it seems thicker, gently roll it with a rolling pin. (Be careful of rolling too thin or the pastry tends to crack when cut or served.)
3 Fold in the edges: Dip your fingers into flour to prevent them from sticking and fold in the edges all around the pastry to make a 1/2-inch border. Press the tines of a fork into the border to seal and make a decorative crust; dip the fork in flour as needed to prevent sticking.
Finally, prick the pastry and the border all over using the fork to prevent the pastry from puffing up during cooking.
4 Prepare the potatoes: Cut the potatoes 1/8-inch thick (or thinner) using a mandolin or a chef’s knife.
Spread the potatoes evenly over the pastry, overlapping slightly. Don’t worry about making it look perfect; it should have a rustic feel. Sprinkle with olive oil, salt, pepper, and 1 tablespoon of the herbs.
5 Bake the tart: Transfer the baking sheet to the oven and bake the tart at 375°F for 35 to 40 minutes. Check the tart halfway through cooking, and if there are big puffs along the edge or bottom, prick them with the tip of a paring knife so they deflate.
When done, the pastry should be golden at the edges and the potatoes are tender when pierced with a skewer.
6 Serve the tart: Slide the tart onto a large cutting board and sprinkle with the remaining 1 tablespoon of fresh herbs and a few pinches of salt. Slice into 12 pieces. Serve with sour cream for dipping or dolloping on top.