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1
First boil potatoes in salted water for about 10 minutes; drain.
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2
Make the caramalized onions next. Melt butter in a medium skillet over medium heat. Add onions, 1/4 tsp salt, dash pepper and cook, stirring often, until onions are very soft and deep golden brown, about 12 to 15 minutes. Stir in vinegar and sugar and cook until onions are covered in a glaze, about another minutes. Cover and keep warm.
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3
Cut the potatoes into 1/2 inch slices, then coarsely chop.
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4
Heat oil in a large oven proof skillet over medium heat. Add potatoes, 1/2 teaspoon salt and 1/4 teaspoon and cook, uncovered, until potatoes are lightly browned on all sides about 12 to 15 minutes.
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5
Preheat the broiler.
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6
In a medium bowl, whisk together eggs, rosemary and about 1/2 teaspoon salt and 1/4 teaspoon pepper until well combined. Pour over potatoes in skillet and turn heat down to medium low. Once set, lift up the frittata at one side and tilt the pan to let uncooked egg get under cooked part. Cook until the top is almost set, about 5 minutes. Sprinkle with gorgonzola cheese, then put the skillet under the broiler until the top is puffed and set, about 1 minute.
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7
To serve, spread the caramelized onions on top of the frittata and cut into wedges. Can be served either hot or warm.
