Mostarda di Cremona isn't actually a mustard, rather it's summer fruits preserved in mustard-infused syrup. Use a mix of peaches, pears, plums, apricots, nectarines, quince, cherries, citrus rind. Makes for a beautiful holiday gift.
I've seen recipes that add too much sugar, or throw in spices that at best turn it into sweet ketchup and at worst a muddy jumble of flavors. To my mind this is a terrible crime against the naturally beautiful flavor of the tomato.