Stir together all ingredients except sugar in a bowl.
Cook sugar in a dry heavy saucepan over moderate heat, undisturbed, until it begins to melt. Continue to cook, stirring occasionally with a fork, until sugar is melted into a colored a golden caramel. Tilt pan and carefully pour in hoisin mixture (caramel will harden and steam vigorously). Cook over moderately low heat, stirring, until caramel is dissolved and sauce is thickened, 6 to 8 minutes. Cool to room temperature.
Should you feel it needs a little more balance with the sweet add additional lemon juice.