Mix the ground beef, Dijon mustard, capers, egg, salt and pepper in a bowl. Divide the mixture into quarters and shape each quarter into a patty. Each patty should be slightly larger than the bread slices.
Place the patties on the bread slices, pressing them down well and folding the edge of the the meat patty over the edges of the bread slices. Season with salt and pepper.
Melt a knob of butter in a frying pan and fry the Pariserbøf patty side down for 5-6 minutes. Turn the patty over and fry it on the bread side until the bread browns and turns crispy.
Turn the Pariserbøf over once more for 1 more minute and then transfer to a plate. Pariserbøf is generally served medium to well done, although it can be served medium-rare as well if you omit the eggs or use pasteurised eggs for safety.
Garnish the Pariserbøf with sliced, onion, capers, horseradish, beetroot, and top with an egg yolk each.
Serve immediately and enjoy!