In a small pan, melt the 1 tablespoon of butter. When the foam subsides, add the onions and cook for 2 or 3 minutes or until they are soft and transparent, but not brown. Scrape into a large bowl and add the meat, egg yolks, capers, salt, pepper and white vinegar. Mix together and moisten with the heavy cream. Then stir in the drained chopped beets. Shape the mixture into 12-14 round partties, about 2-3 inches in diameter. In a heavy large skillet, heat the butter and oil. When the foam subsides, add the patties 3 or 4 at a time and cook over moderately high heat for 5 to 6 minutes on each side or until they are a deep brown. In Sweden, these spicy hamburgers are frequently served with a fried egg set atop each, in which case they are made larger and thicker.