You can use whatever onions you like or have on hand. I have made them with large white onions, yellow or even red onions. I have used different onions depending on the flavor I want in the cheesy meatballs. White onions tend to give flavor but have more of a bite, with yellow onions giving a little more subtle onion flavor that is sometimes considered a sweeter option. Red onions tend to give a very strong specific flavor, which is perfect if you like a strong flavor.