candied fruit in a mustardy sauce
Not mustard, which is senape, but candied fruit in a mustard syrup. 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 Ark of Taste.