2 tablespoons golden raisins
1 pound onions, peeled
4 tablespoons extra-virgin olive oil
4 tablespoons butter
¾ teaspoon salt, or more
3 bay leaves
2 tablespoons red wine vinegar
1 pound chicken livers
freshly ground black pepper, to taste
Put the raisins in a bowl with warm water to cover, so they plump up for a few minutes.
Slice the onions in half and then crosswise into 1/4-inch thick half moons. Put 2 tablespoons of oil and 2 of butter in one of the skillets and set it over medium heat. As the butter melts, stir in the onions, sprinkle with 1/2 teaspoon salt, and drop in the bay leaves. Shake the pan and turn the onions as they heat and start to sizzle, then lower the heat slightly and cook the onions slowly as they gradually wilt and start to color, stirring and tossing them occasionally.
After 15 minutes or so, when the onions are deep gold all over, raise the heat a bit, pour in the 2 tablespoons of vinegar and cook for a minute, stirring the onions in the vinegar as it steams and evaporates. Drain the raisins and gently squeeze out the excess liquid; scatter them in the skillet and toss together with the onions. Now lower the heat and continue cooking the onions until they're as dark and caramelized as you want them be. Turn off the heat and let the onions rest in the hot pan.
While the onions are cooking, clean the livers (I do this on paper towels): with a paring knife remove all the fat, veins and membranes and slice the livers into separate lobes. Rinse them well then pat dry with fresh paper towels. Sprinkle 1/4 teaspoon salt and grind pepper on all surfaces of the livers.
Heat the remaining 2 tablespoons of olive oil and 2 of butter in the second skillet, over medium-high heat, until the butter is melted and foaming. Before the butter begins to color, lay the livers in the pan, without crowding them. With the heat high, cook the livers for 2 minutes or a bit more, until they're browned and crisped on the underside, then turn them over (in the order in which you put them in the pan). Cook about 2 minutes on the second side until they are nicely crisped all over and lift them-a couple at a time-with a spider or slotted spoon, let the oil drain off and place them in the first warm skillet, next to the onions. Don't overcook the livers: they should still be slightly pink inside when you move them. Taste a piece and season with more salt and pepper if you want.
If you will be serving right away, heap the onions on top of the livers, then spoon onions and liver together onto warm serving plates. If serving is delayed, leave the onions and livers in separate parts of the pan so the livers don't get soggy. Smother the livers with onions when you serve.