Wash the spinach well and blanch it for a few minutes in boiling, slightly salted water, drain well.
Put the minced meat in a mixing bowl, pulse the stale bread in the food processor and add the breadcrumbs to the meat together with the grated parmesan, add the egg and a pinch of salt and mix.
Squeeze the spinach very well to get rid of all the water and then chop it very finely and add to the meat mixture. Mix it all with your hands until you have a smooth, very well-combined mixture, and add a little salt. Then make small, cigar-shaped meatballs.
Put the oil into a deep frying pan and add the tomato passata, then lay the meatballs in the sauce one next to the other in a single layer. Cover and simmer gently for about 30 minutes until the meatballs are cooked; check every so often to see the sauce does not completely dry out and if necessary, add a ladle of water. By the end of cooking, the meatballs should have absorbed nearly all the sauce.