Wash the beef heart, put it in a pot, cover with water and add 2 tablespoons of salt. Bring to the boil. Clean, wash and coarsely chop the soup greens and add to the beef heart. Cook everything over medium heat for 3 hours.
Peel and wash the carrots and add them to the meat 15 minutes before the end of the cooking time. Remove the beef heart. Pour stock through a sieve and measure 3/4 litre. Put whole carrots aside. Peel onions and cut into slices.
Heat the oil in a pan, fry the onion slices in it, remove. Add flour to the pan and fry until golden brown. Add fat and mix. Deglaze with stock and cream while stirring and bring to the boil.
Season to taste with vinegar, salt, pepper and sugar. Remove skin and tendons from the beef heart. Cut meat into large cubes and add to the sauce. Cut whole carrots into slices with a colored knife.
Add carrot slices and onion slices to the meat. Heat everything once again. Pour the ragout into a bowl. Wash parsley, chop finely and sprinkle over the ragout. Serve with mashed potatoes and lettuce.