In a bowl using your hands mix together the beef, potato, egg yolk, salt and pepper and just enough milk to hold the mixture together without making it too sticky to shape. If you have a food processor work the mixture in it to make it smoother.
Mix in the beetroot, onion and capers. Divide into four portions and shape each one into hamburger shapes about 2 cm thick.
Fry in the butter and oil mixture until nicely browned on both sides. Serve as they are or with a fried egg on top.