Zia Tea, my Italian aunt, used to make meat balls when she had left over meat which did mot make another meal for the family. She stuffed it in her meat grinder, ground it and then added raw ground meat. She was not wealthy in terms of money, so she had plenty of fruit, veggies and herbs from her wildly growing garden, but could not afford to buy lots of meat and especially no costly cuts of meat. My mother, studying in Rome during the late 1950s and early 1960s, used to spend her holidays with zia Tea, whose husband worked as a fisher, and they were quite poor but shared everything they had with my mother. The meat they occasionally had was cheap horses' meat and the fish mostly salted codd which lay around in the kitchen cupboard. When I was a child, zia Tea and her husband run a pet store and had some more money, but not much. No more horse meat and salted codd, but some inexpensive beef or pork cuts, thinly sliced turkey or sometimes one of the rabbits from the pet store. Stocking up the left over meat which she ground for polpettine, she used whatever was on sale at the local butcher's, so it was never exactly the same polpettine. But they were the best I ever had in my life!
You can use every kind of ground meat you like and adapt the amount of garlic to your taste. She didn't use a lot so that the lemony flavor and the sage and celery were not overwhelmed.