In a large and deep bowl, beat the egg and milk. We add the slice of sliced bread and mash it with a fork, while soaking it in the liquid ingredients. Season with a clove of garlic, peeled and finely chopped, as well as with a little salt. Add the minced meat and the potato, peeled and grated, and mix well until a homogeneous dough is obtained.
Next, we cover the base of a plate with flour. We take small quantities of the seasoned meat and we roll them lightly, then depositing them on the flour. We shake the plate with circular movements, allowing the meatballs to turn and are covered in flour. We remove them to another plate and repeat the operation as many times as necessary until we finish with the meat.
Heat plenty of oil in a pan and fry the meatballs for a couple of minutes. We just want let them brown on the outside, so we remove them immediately and transfer them to a large casserole. We reserve until we have the sauce ready.
To make the sauce, peel and chop the other clove of garlic, and also the onion, in brunoise. We heat a little olive oil in a frying pan and the poach over low heat for about 10 minutes. After this time, we add a tablespoon of flour, stir and, when it begins to take color, add the meat broth.
We let let the sauce thicken and cook for a few minutes before sprinkling with it the meatballs that we have in the other casserole. We place the casserole on the fire and cook over a very low heat, with the lid on, for 20 minutes before serving.