In a large skillet cook the bacon over moderate heat until it is crisp, transfer it to paper towels to drain, and pour off all but 2 tablespoons of the fat. To the fat remaining in the skillet add 2 tablespoons of the butter and cook the onions in the fat over moderate heat, stirring occasionally, until they are softened and light golden. Add the apples, cored and cut into eighths, and cook the mixture until the apples are light golden. Add the vinegar, the sugar, and the wine, bring the mixture to a boil, and boil it for 3 minutes, or until it is thickened. Transfer the mixture to a heated platter.
In the skillet, cleaned, heat the remaining 2 tablespoons butter over high heat until the foam subsides and in it saute the liver, patted dry, turning it, for 3 to 4 minutes, or until it is browned and slightly pink within. Arrange the liver over the apple mixture, sprinkle the bacon, crumbled, and the chopped parsley over it, and garnish the platter with the parsley sprigs and the lemon wedges.