Not mustard, which is senape, but fruit (a mix of grapes, quinces, plums, pears, apples, peaches, tangerines, and cherries) boiled in a simple syrup, with mustard seeds or powder. Cremona in Lombardy is especially famous for it. It's a classic sauce with bollito misto, and pairs well with strong cheeses. In Piedmont, they prefer mostarda d'uva: a mustard made with grape must.
Mostarda di Mantova (or mostarda di mele campanine or mostarda mantovana) is made from little green apples called mele Campanine.
The mostarda di anguria bianca made around Mantua, is in the Slow Food Presidium.
Image by Silvio