Put the head of the lamb in a big pot and bring it to boil. Once it boils, discard the water, wash the pot, put fresh water in and add the lamb head and the other bones in. Boil for about 1.5-2 hours, depending on how big the head is.
Pick up the foam that forms on top of the pot whilst the bones are boiling. This is so that you get a clear soup in the end.
After 1.5 hours, add the chopped vegetables, except for the spring onion. Let the pot boil for another 15 minutes or so, until the vegetables are cooked.
Add salt to taste, the bors, the rice, and the spring onions. Boil for another 15 minutes.
Meanwhile, start making the dressing. Mix the egg yolks with the sour cream. Then start adding small quantities of the soup, slowly, whilst mixing continuously. Do this until the sour cream mixture reaches the same temperature as the soup. It will take quite a bit of soup, so make sure you are using a large enough bowl.
Turn the heat off, and dress the soup with the sour cream and egg mixture. Add chopped lovage on top. If you don't have fresh lovage, you can always add dried one.
Serve hot alongside crusty bread, pickled chili peppers and/or garlic sauce.