The initial blanching of the onions insures that they'll be tender when cooked through. If the raisins are hard, pour boiling water over them and let stand for 5 minutes. Drain and proceed with the recipe.
Working in 2 batches, blanch the onions in a medium pot of boiling water for 3 minutes. Peel.
Heat the oil over medium heat in a large skillet. Cook, turning the onions over as they brown, until almost tender, about 15 minutes. Add the broth, vinegar, sugar and raisins and bring to a boil. Reduce to a simmer and cook until the sauce is slightly thickened and the onions are tender, 3 to 5 minutes. Stir in the salt and serve.