Russet or baking potato, 1, peeled, halved lengthwise, and thinly sliced
Yellow onion, 1 small, halved and thinly sliced
Frozen puff pastry, 1sheet, thawed and cut into four 5-inch (13-cm) squares
Gruyère cheese, 1cup (4oz/125g) shredded
Make the filling
Preheat the oven to 400°F(200°C). In a bowl, stir together the oil, rosemary, ½teaspoon salt, and ⅛teaspoon pepper. Add the potato and onion and toss well.
Prepare the pastry
Lay the pastry squares on a baking sheet. With a sharp paring knife, make a cut ½ inch (12 mm) from the edge all the way around each square, being careful not to cut more than halfway through the pastry. Inside the border, prick the pastry all over with a fork.
Fill and bake the tartlets
Sprinkle 2tablespoons of the cheese inside the border of each pastry square. Divide the potato and onion mixture among the squares and sprinkle with the remaining cheese. Bake until the borders of the tartlets are puffed and brown and the potatoes are tender, about 20 minutes. Serve warm or at room temperature.