In a mixing bowl, combine the ground beef, egg, egg yolks, Dijon mustard, finely chopped capers, salt, and pepper. Mix until all ingredients are well incorporated.
Heat 20g of butter in a frying pan over medium heat. Once melted, add the beef mixture. Cook for about 5-7 minutes, breaking it apart with your spatula until browned and cooked through.
In the same frying pan, add the remaining 20g of butter and toast the sourdough slices until golden brown on both sides.
Place a generous amount of the cooked beef mixture on each slice of toasted sourdough. Top with sliced red onions, diced pickled beetroot, drained capers, and freshly grated horseradish.
Season with additional salt and pepper if desired, and serve immediately.