In a bowl pour the ground meat and add the egg, Parmigiano cheese, chopped parsley and a little salt. Mix well until the mixture is smooth.
Add the milk and mix. Finally add the breadcrumbs a little at a time. Incorporate into the mixture while keeping on stirring. You should get a firm but soft mixture.
You can start forming the meatballs. Round some of the mixture with your hands and place it on a plate. Take care to create meatballs of the same size: about 2" or 3" in diameter.
Now pour the olive oil into a frying pan. Sauté the chopped onion over low heat until golden brown.
Add the meatballs one at a time and let them brown for a few minutes, turning gently so they are browned on all sides.
Deglaze with white wine and allow the alcohol to evaporate.
Add the tomato passata and the tomato paste, dissolved in hot water. Stir gently being careful not to break up the meatballs.
Cover with a lid and simmer for about 20 minutes. Before serving, taste and add a little bit of salt if necessary
