Prepare the meatballs. Add all of the meatball ingredients (ground meat, dill, parsley, both types of onions, egg, and 3½ tablespoons of rice) to a mixing bowl, season with salt and pepper, and knead to combine.
Refrigerate for 30 minutes to set.
Bring 2 liters of hot water or chicken stock to a boil in a cooking pot. Add the bay leaf.
Shape into 9 round meatballs and dip them one by one into the boiling liquid. If the meatballs are too sticky, you can oil your hands to handle them. You can also use a spoon to safely dip them into the hot liquid.
Simmer over moderate heat, covered, for 15 minutes.
Add the celery, carrots, potatoes, zucchini, and 5 tablespoons of rice.
Simmer with the lid slightly open on the side for another 30 minutes or until the rice and potato are completely tender. Take the pot off the heat for 5 minutes.
Lightly beat the eggs with a whisk and pour in the lemon and lemon zest.
Ladle the hot soup (only the liquid) into the eggs very slowly, threadlike, while whisking the eggs constantly. Add at least ⅓ of the soup, then return everything to the pot. Stir gently.
Thicken the soup. Return the pot to the lowest heat for a couple of minutes for the soup to thicken even further, or skip this step and simply allow the soup to sit on the warm stove (with the lid slightly open) for at least 20 minutes before serving. The more it cools, the more it thickens. It gets very creamy when reheated.
Serve with an extra squeeze of lemon if you want, and freshly ground pepper.