Heat 1 tablespoon butter in a small skillet over moderate heat and saute the onion until tender but now brown, about 5 minutes.
3 tbsp butter, 1/2 medium onion
Combine with the ground beef, egg yolks, cream, beets, capers and vinegar in a bowl and stir to combine.
1 lb ground beef, 1 tbsp cider vinegar, 1/2 cup heavy cream, 4 large eggs, 1 tbsp capers, 1/4 cup beets
Form into 12 to 16 small patties (or 4 to 6 larger patties if you plan to serve them topped with fried eggs).
Heat the remaining 2 tablespoons of butter and oil in a large heavy skillet over moderate heat and fry the patties until cooked through, about 4 to 6 minutes per side.