1. Preheat oven to 350ºF. Heat oil in a cast iron skillet with ovenproof handle over medium heat. Add onions, 1/2 teaspoon salt, and 1/8 teaspoon pepper; cook, turning with tongs, 10 minutes, or until well browned. Stir in vinegar and cook 1 minute longer. Stir in potatoes, greens, bacon, and 1/2 teaspoon salt and 1/8 teaspoon pepper.
2. Pour eggs over vegetables and stir to blend. Cook over medium heat until mixture begins to set, 3 minutes. Place skillet in oven and cook until set, 15 minutes. Let stand 5 minutes. To serve, loosen around edges with spatula and turn out onto platter.
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