1. If using fresh corned beef, do the advance prep: Place the corned beef in a large pot, add water to cover, and bring to a boil, then reduce the heat to low and cook at a bare simmer until a sharp knife will pass all the way through without much resistance, about 2½ hours. Drain well, then, as soon as the meat is cool enough to handle, chop it fine. Set aside.

2. Meanwhile, preheat the oven to 350°F. Pierce the sweet potatoes in a few places with a fork and bake until just tender, about 50 minutes. As soon as they are cool enough to handle, peel and dice small. Set aside.

3. Make the onions: In a large sauté pan, heat the oil over medium-high heat until hot but not smoking. Add the onions, sugar, wine, and vinegar and cook, stirring occasionally, until most of the liquid has evaporated and the onions are golden brown, 30 to 40 minutes. Season well with salt and pepper, stir in the currants, and set aside.

4. In a large bowl, combine the corned beef, sweet potatoes, and salt and pepper (use plenty). In a large sauté pan, heat the olive oil over medium-high heat until very hot but not smoking. Add the meat mixture, spreading it out to cover the entire bottom of the pan. Press the hash down evenly with a large metal spatula, reduce the heat to medium, cover the pan, and let the hash cook undisturbed for 15 minutes. Uncover the pan, turn the hash over in sections with the spatula, and raise the heat to high. Cook for 5 more minutes, turning the hash every once in a while with the spatula.

5. Serve the hash hot, covering each portion with a generous layer of the caramelized onions.