Tear the bread into a bowl and pour the milk over it. Allow it to sit for a few minutes, then add the beef, pork, egg, parsley and Parmesan. Add some salt and pepper and mix it all up with your hands. You can do this ahead of time and keep the mixture in the fridge.
To make the sauce, heat a drizzle of oil in a large, heavy-bottomed pot set over medium-high heat, and cook the garlic for a minute or two. Pour in the tomatoes, add the onion and butter, give everything a stir and a good pinch of salt. Reduce the heat to low and simmer for 20-30 minutes, while you shape the meat mixture into 1-2 inch balls. Set them aside on a baking sheet.
To cook the meatballs, carefully drop them into the simmering sauce. You might not fit them all into your pot - that's ok. Leave the rest to simmer after the first has been cooked, or freeze them for another batch.
Cover the pot with a lid and leave it to simmer. Don't stir - you don't want to break your meatballs apart before they set - but Luisa says you can shake the pot a little. After 20 minutes, they should be cooked through. (If you need to cook more, remove them to a bowl with a spoon and simmer your next batch in the remaining sauce, then return the first batch to the pot when you're ready to serve them.)
Plunk the pot on the table (on a trivet or folded towel, of course) and serve with spaghetti and extra Parmesan.