Butter 6 individual metal tart pans with removable bottoms. Set aside.
Using a rolling pin, roll out puff pastry to 1/8-inch thickness. Using wide part of a tart pan as a guide, cut out six pastry circles. Prick each circle several times with a fork. Lay on parchment paper-lined pans or cookie sheets. Refrigerate for 30 minutes.
In a sauté pan over medium heat, heat the olive oil. Add onion slices and cook, stirring occasionally, until golden and caramelized. Be careful not to burn.
Arrange two or three caramelized onion slices on bottom of each tart pan. Sprinkle generously with crumbled gorgonzola cheese. Layer potato slices over cheese. Sprinkle with salt and pepper. Repeat, adding another layer of onion, gorgonzola, potato slices and sprinkle with salt and pepper. Place one circle of chilled puff pastry on top of each tart.
Bake at 400° until tart tops are puffed and golden, about 20 minutes. Remove from oven, invert tarts onto a plate and serve.
Nutritional Analysis per serving: 299 calories, 10 g protein, 15 g fat (46% calories from fat), 31 g carbohydrates, 21 mg cholesterol, 4 g fiber, 501 mg sodium.