Heat the butter and oil in a frying pan (skillet). Cook the onion slowly until pale and soft, then allow to cool
Combine the minced beef, egg yolks, capers, sea salt and pepper.
Add the onion and mix thoroughly together
Add the beetroot and juice (or relish), mix well.
With wet hands, shape the mixture into 4 large flat patties (the traditional shape, or 8 smaller taller patties.
Heat a chargrill or pan grill, oil it and the patties lightly and cook them on a medium heat for about 5 minutes each side (alternately heat a little oil in a frying pan and cook them that way).
Split and toast the muffins, serve the burgers in the toasted muffins with a little salad of tossed green leave or with a little wilted spinach and boiled new potatoes. I added an extra spoonful of Beetroot Relish on top of the burger.
Serve on English Muffins with a salad of tossed green leaves or wilted spinach, and small boiled new potatoes.