1. In a small skillet, melt 1 tablespoon of the butter over moderate heat. Add the onions and, stirring frequently, cook for about 5 minutes, or until they are soft and transparent but nbt brown.
2. Watch carefully for any sign of burning and regulate the heat accordingly. Pour in the wine and wine vinegar and, stirring constantly, bring to a boil over high heat.
3. Cook briskly, uncovered, until the liquid is thick and syrupy. Remove the pan from the heat.
4. Immediately melt the remaining 2 tablespoons of butter over high heat in a heavy 10- to 12-inch skillet. When the foam begins to subside, add the kidneys and liver.
5. Turning the pieces of meat about constantly with a spoon, saute for 2 to 3 minutes, until they are lightly and evenly browned.
6. Stir in the reserved sauce and, still stirring, cook for 60 seconds longer. Add the salt, taste for seasoning, and transfer the entire contents of the skillet to a heated platter.
6. Sprinkle the kidneys and liver with the parsley and serve at once.
