Frikadeller are a quintessential Danish tradition. They are so commonly eaten that supermarkets sell mixed packs of pork and beef mince for making them at home. Everyone has their own recipe, and different ideas on what makes them work. In the South of Denmark, some people add sparkling water to the mix instead of milk to make them lighter and puffier (I have no idea if this works, it sounds like an old wives tale).
You can add in different things to flavour these or just keep them plain and traditional. The mustard sauce is not in any way traditional, it is meant to be served with gravadlax, but I have a debilitating mustard addiction (if by debilitating you mean regular sneezing due to eating too much mustard) and it does complement the pork nicely. —Canalcook