Into a large enough bowl to hold everything, put all of the ingredients EXCEPT the beef. Stir with a spoon or large fork until the ingredients are incorporated and there are no streaks of egg or cheese, etc.
Add the beef and, using your hands, gentle mix the beef and other ingredients until pretty evenly disbursed. Don't smoosh roughly or tamp it down.
Roll the meat mixture into balls (I use a quarter-cup aka 2 oz. cookie scoop for uniformity) and set aside while you heat the oil.
In a cast iron (or other heavy-bottomed) skillet, pour the olive oil and set over medium-low to medium-high heat; whatever it takes to just get a shimmer because olive oil has a very slow smoke point.
Place about a third of the meatballs (or as many as will fit without crowding) into the pan and fry until browned on the bottom. The meatballs will release when they're ready. From that point, turn every two minutes or so, until they are browned evenly on the outside for approximately 12 minutes per batch at this size.
For them to be consumed safely, an instant-read thermometer should register 165F at the center. I don't worry that much about center temp when I am putting them directly into sauce to simmer.
When they are done, set on a cooling rack over paper towels to drain and continue until all of the meatballs are cooked.
Once done, they can be put into simmering tomato sauce and served immediately, after soaking up the sauce for a bit (or up to an hour), or plain for the purist.
To make ahead, cool the completely cooked meatballs to room temperature and either seal and place in the refrigerator for a day or two or seal in freezer bags and stow in the freezer for long-term storage.