Place alternating layers of fruit and sugar in a steel pot, then let it rest for 24 hours.
Place the pot over medium heat and boil for 5 minutes. Turn off the heat and let it rest for 24 hours in a dry place. Do not cover the pot when boiling or resting, to prevent condensation from dripping on the fruit.
Boil for another 5 minutes and let it rest, as before, then repeat a third time and let it cool.
Put 5 drops of mustard essential oil in canning jars and fill them with the fruit and syrup, close tightly, and store in a dark place. Allow the mustard to sit for a few days before being used.