Ingredients:

1. To make the caramelized onions, in a medium nonstick skillet, melt the butter over medium heat. Add the onions, salt, and pepper. Cook, stirring often to avoid scorching, until the onions are very soft, 12 to 15 minutes. Stir in the vinegar and sugar and cook until the vinegar is reduced to a glaze, about 1 minute. Keep the onions warm. (The onions can be prepared up to 3 days ahead, covered, and refrigerated. Reheat before using).
2. Bring a large pot of salted water to a boil over high heat. Add the potatoes and cook for 10 minutes, until parboiled. Drain and rinse under cold water until cool enough handle. Cut into ½ inch-thick slices, then chop coarsely.
3. In a 9 to 10 - inch nonstick ovenproof skillet, heat the oil over medium heat. Add the potatoes, ½ teaspoon of the salt and ¼ teaspoon of the pepper. Cook, uncovered, turning the potatoes occasionally, until they are browned and tender, 12 to 15 minutes. Spread the potatoes as evenly as possible in the skillet.
4. Position the broiler rack about 6 inches from the source of heat and preheat the broiler. In a medium bowl, whisk the eggs, rosemary and the remaining ½ teaspoon salt and ⅛ teaspoon pepper until well combined. Pour over the potatoes and reduce the heat to medium-low. Using a rubber spatula, lift up the cooked part of the frittata, and tilt the skillet to allow the uncooked eggs to run underneath. Continue cooking, occasionally lifting the frittata and tilting the skillet as described, until the top is almost set, approximately 5 minutes. Sprinkle with the Gorgonzola. Broil until the frittata is puffed and the top is set, approximately 1 minute.

Serves 6