Heat olive oil in a large skillet over medium heat.
Add onion. Cook 3-4 minutes, stirring occasionally.
Add garlic and parsley. Cook an additional 3-4 minutes until onion is translucent and garlic is cooked.
Remove from heat and set aside.
Heat oven to 350 degrees.
Brush two sheet pans with vegetable oil. Make sure they're well coated. Set aside.
Place ground beef and ground chicken in a large bowl.
Add all remaining ingredients, including the mixture from skillet.
Stir to combine everything together in the bowl. Stop as soon as all ingredients are incorporated to avoid the meatballs getting tough.
Take a small amount of the meat mixture and roll it into 2” balls. (Feel free to make them smaller if using for appetizers.)
Place meatball on one of the greased pans.
Continue doing this - place so the meatballs are tight together but not touching - until all of the meat mixture is used up.
Place in preheated oven and bake for 25-30 mins.
Remove meatballs from oven and allow to cool.