How to Make Mostarda - A Wonderful Condiment Flor Bland Foods

Set a colander in a pot and upend the bowl into it. Drain the fruit well and return it to the bowl. Heat the syrup over a medium flame, reduce the heat some when it comes to a boil, and continue boiling it until its volume is reduced by half. Skim the foam from the surface with a slotted spoon or strainer, and pour the syrup back over the fruit.

Two Things:
The procedure outlined here is based on a chef's demonstration on the Gambero Rosso channel. He concentrated the syrup considerably, and separately from the fruit. Other recipes I have found concentrate the syrup much less, as little as 5 minutes, and some have you boil the fruit with the syrup. The choice is up to you.

Some thoughts about Color: In commercially prepared Mostarda di Cremona the syrup is invariably colorlss and crystal clear, regardless of the fruit that goes into it. This is in part because some of the coloring substances are in the skins, which are removed, and in part because they are careful not to overheat and thus caramelize the sugar. Or (I suspect) they use a fresh batch of syrup when they put the fruit in the jar.