Caramelized Onion, Serrano Ham, Gruyère and Grape Puff Pastry Tart

You can use this basic recipe, originally developed by Copia Senior Culinary Instructor Jill Hough for Bon Appetit magazine, year-round—mixing up the toppings depending on the season. This is a version we serve in the fall, when grapes are being harvested.
The pastry and toppings can be prepared in advance, then quickly assembled right before cooking and serving. Or the whole tart can be made in advance and served room temperature. The thing to remember is that the cook time isn’t long enough to do much besides melt the cheese and heat the toppings. So whatever you put on the pastry should be either pre-cooked (roasted, blanched or sautéed) or something that you don’t mind eating relatively raw, like thinly sliced onions.

Serves 8 as a light entrée or appetizer

2 tablespoons unsalted butter
2 tablespoons olive oil
2 large yellow onions, sliced
Salt
Freshly ground black pepper to taste
1 sheet (half of a 17.3-ounce package) frozen puff pastry, thawed
1 1/4 cups grated Gruyère cheese (about 4 ounces), divided
1 ounce thinly sliced Serrano ham or prosciutto, cut into thin strips
1 cup halved seedless grapes (red, green or a combination)
1 tablespoon chopped fresh rosemary

In a large skillet over medium-high heat, heat the butter and olive oil. Add the onions and cook, stirring occasionally, until translucent, about 5 minutes. Reduce the heat to very low and cook, stirring occasionally, until the onions are deeply golden, 45 minutes to an hour. Set aside to cool; add salt and pepper to taste.

Line a baking sheet with parchment. On a floured surface, roll out the puff pastry to a 10-inch square; transfer to the prepared sheet. Using a fork, pierce the pastry all over. Chill 20 minutes.

Preheat an oven to 400°F. Sprinkle 3/4 of the cheese on the pastry, leaving a 1-inch plain border. Arrange the caramelized onions, Serrano ham and grapes on top. Sprinkle with the rosemary and the remaining cheese. Bake the tart until golden, about 25 minutes. Serve warm or at room temperature.