1) Put the potatoes in a medium saucepan and cover with cold water by about 5 centimeters. Season with salt. Bring to a boil, lower the heat, and simmer until just tender, about 6 minutes. Drain and set aside.
2) Cook the bacon in a large cast iron skillet over medium heat until crisp, about 4 to 5 minutes. Transfer to a kitchen towel-lined plate. Pour all but 1 tablespoon of the bacon drippings into a small bowl and set aside.
3) Add the onion and cook, stirring, until golden brown, about 8 minutes. Transfer to a bowl.
4) Wipe out the skillet and return to medium-high heat. Heat the remaining bacon fat and 2 tablespoons vegetable oil in the skillet. Add the potatoes, season with salt and pepper to taste, and arrange in a single layer. Cook, without stirring, until well browned on 1 side, about 4 minutes.
5) Using a spatula, turn the potatoes in small sections, and continue cooking, turning occasionally, about 6 minutes or until well browned. Stir in the onions, heat through, and sprinkle with the parsley and cayenne, if desired.
6) Transfer to a warm platter and crumble the bacon on top, serve.
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