Saute onion on medium-low heat first to soften it up. In large bowl, beat eggs *OR* pour in oil. Stir in bread/Rice Krispie crumbs until well combined. Stir in milk, onion, parsley, salt, mustard. Add meat and knead with wet hands until well blended.
Form meat into balls and place on large greased cookie sheet. Bake in 400 °F oven for about 25 minutes or until browned.
With slotted spoon, transfer meatballs to trays, spacing balls so they are not touching. Let cool, then freeze until firm. Package in heavy plastic bags or other container and store in freezer.
Bread crumb can be replaced by crushed Rice Krispies type cereal. Be sure the crumbs are safe for your needs.
Use any type of safe milk substitute.
1 Tbsp dry parsley can be substituted in place of the fresh parsley.
Ground turkey or chicken can be substituted in place of the ground beef.