Meanwhile, in a medium saucepan, heat the oil. Add the onion and cook over moderately high heat, stirring, until golden, about 4 minutes. Add the dates and balsamic and sherry vinegars and cook over low heat until the dates are softened and the liquid is syrupy, about 4 minutes. Transfer to a bowl and let cool. Stir in the goat cheese and season with a generous pinch of pepper.