1. Prepare the broth by placing all the ingredients and the water in a pressure cooker. Seal the lid and cook under pressure for 30 minutes. If using a regular pot, simmer gently for 1½–2 hours, topping up with a little water if necessary. Strain the broth and set it aside.
2. Prepare the meatballs by adding all the ingredients, except for the broth, to a bowl. Knead well, and finally add enough broth to create a soft mixture that won’t fall apart when shaped. Shape the meatballs and place them on parchment paper. At this stage, the broth can be replaced with water without significantly affecting the final flavor.
3. Pour the broth into a deep pot and season lightly with salt and pepper. Bring it to a boil and add the meatballs one by one, carefully. Cook for about half an hour. Taste one meatball to check if it’s done. Adjust the salt if necessary, then remove the pot from the heat.
4. To prepare the egg-and-lemon sauce, dissolve the cornstarch in a little cold water. In a bowl, whisk together the egg yolks, the lemon juice, lemon zest, and the dissolved cornstarch until smooth. (The cornstarch not only helps thicken the sauce but also reduces the risk of the egg-and-lemon sauce curdling.) Gradually whisk a few ladles of the hot broth into the egg mixture to temper the eggs.

Next, in the pot—which we’ve removed from the heat—we gradually add the egg-and-lemon mixture and swirl the pot by its handles with both hands in a circular motion so that it spreads evenly throughout. Once the egg-lemon mixture is fully incorporated into the soup, return the pot to low heat and let it sit for a few seconds while stirring carefully with a ladle, until you feel the first bubbles of boiling begin to form. Remove from the heat immediately and let the soup cool slightly. Add freshly ground pepper and serve.