Peel the garlic. In a large skillet, heat 2 tablespoons of the olive oil. Add the onion and cook over moderate heat until softened, about 5 minutes. Add the apple cider and boil over high heat until reduced to 1/4 cup, about 4 minutes. Add the vinegar, roasted garlic and the remaining 2 tablespoons of olive oil and season with salt and pepper.