Finely chop the onion. Heat the butter and olive oil in a fry pan and cook the onion slowly until pale and soft, then cool.
Combine the minced beef, egg yolks, capers, sea salt and pepper in a bowl, add the onion and mix together thoroughly. Add the pickled beetroot and juice, and mix well. With wet hands, shape the mixture into 4 large, flat burgers (the traditional method) or make 8 s