Pat the matjes dry and take the effort to remove most of the bones with tweezers. Cut the matjes, beetroot and onions into small cubes of the same size. The finer it is diced, the better the flavors can combine. Mix the olive oil, balsamic vinegar, mustard, salt and pepper into a marinade. Finely chop the capers. Put the herring, beetroot, onions and capers in the marinade and mix everything very well. Now give the tartare a little rest in the fridge. I`m always brave and prepare the day in advance, but it`s up to everyone.