In a bowl, put the mince, 1 finely grated onion head, the washed rice, salt, black pepper. Separate the egg yolk from the white, add the white to the mince, put the yolk in a small casserole - for the sauce.
Knead the mince well and let it sit as is for at least 30 min. to take in the spices. Chop the remaining 2 onion heads finely, grate the carrot.
Let boil as is for at least 30 min. for the onions to soften well. Before you add the meatballs, salt the water.
Form meatballs from the mince, roll them in flour and put them in the boiling salted water. Once you've added all the meatballs, wait for the water to reach boiling point and time 40 min.
On the 35th min., prepare the sauce. To the yolk in the casserole add the yoghurt and flour, beat well with a whisk so there's no lumps.
Take the pot off the stove and put the casserole in its place, stirring nonstop with a whisk so it doesn't burn. Once the sauce heats up, add a ladle of the broth from the meatballs. Continue stirring and once boiling, return it to the meatballs.
Put on the stove again to boil for at least 5 min. If you don't like the consistency, pour a little of the sauce into a bowl, wait for it to cool, add more flour to thicken and return to the meatballs (not hot). Stir well so there's no lumps and return to the stove to boil until thickened.