A bitterbal is a Dutch snack: a small version of a
kroket, a round ball of
ragout covered in breadcrumbs and deep-fried. The name means bitter ball. I have no idea why it's called that.
A bitterbal never comes on its own, they are served as snacks with drinks and usually arrive in groups of at least five, always with mustard. They are often available in bars, either on their own or as part of a bittergarnituur. This is a plate containing different warm and cold snacks, like blocks of cheese, slices of sausage, perhaps something like chicken nuggets or mini spring rolls, and mustard or chili sauce for dipping. The Dutch version of tapas, if you will.
If you ever eat a bitterbal, be careful. Bitterballen have a reputation for causing blisters on lips and tongue, the filling is very hot!