In a saucepan, melt the butter. Add the onion and cook over moderately high heat, stirring, until softened, 4 minutes. Add the jam; cook until melted and sizzling. Add the dried fruit and its liquid; bring to a boil. Simmer over moderately low heat, stirring, until thickened, 5 minutes. Remove from the heat and stir in the mustard. Season with salt and pepper. Serve warm or at room temperature.