It is important to have at least a 30% fat to meat ratio in minced beef to achieve a juicy and flavorful meatball. If you make the meatballs with minced beef that has a lower fat ratio, you may end up with dry and chewy meatballs. If you can't find this sort of minced beef, you can increase the amount of olive oil you put in the meatball mixture. Also, make sure you chop the onions and garlic very finely. Otherwise, the pieces of onions and garlic will burn when frying the meatballs, and give an unpleasant taste to the dish. Use a food processor or a grater for chopping the onions and garlic, unless you have very good knife skills! Make sure you mix the ingredients very well when preparing the meatballs.