Most recipes for mostarda call for powdered mustard seed, which is much easier to find than mustard oil, and also much safer to work with. You'll want a 2-ounce (50 g) can.
Dissolve it in a cup of wine in a small pot, and gently heat the mixture over a low flame. Stir occasionally, and let the mixture bubble for a few minutes.
While you are preparing the infusion, drain the fruit and concentrate the syrup once again.