Heat the oil and butter in a small frying pan and cook the onion over a gentle heat until soft. Set aside to cool.
In a large mixing bowl combine the beef, egg yolks, capers, salt and pepper. Add the onions and mix well, then add the beetroot and beetroot juice and mix again.
Shape the mixture into 4 large flat burgers .
Heat a griddle pan and lightly oil it. Add the burgers and cook for about 5 minutes on each side. Serve as preferred.