On the Groentenmarkt in Ghent you'll find this wonderful shop that sells its own freshly made mustard. All you have to do is pick out your favorite container and they will fill it from a barrel standing beside the counter. Their mustard is strong and pairs well with meats and cheeses (beer too!), plus it makes a great reminder of your trip when you get home.
Bonus: a cuberdon (local jelly candy) cart is typically nearby the store entrance too.