Make a thin syrup with the sugar and water and the lemon slices, boiling it up in a shallow pan to the point where a drop will adhere to a wooden spoon. Only then throw in the pumpkin cubes. After ¼ hour of vigorous boiling add the mostarda and a few of its chopped fruits (plum, fig, pear, peach, cherry). Boil for a few more minutes. Put into small jars when cool and cover when cold.
The point of the shallow pan is to allow for maximum evaporation aiming at a preserve which consists of almost transparent orange pumpkin cubes in syrup. The mostarda di Cremona contains mustard oil which is just what the pumpkin needs to give it a little bite. If unobtain-able, one can used preserved ginger, chopped with some of its syrup.