Heat oil in a pan over medium heat. Fry the finely chopped onion for 3 minutes until softened. Add the finely chopped garlic and cook for 1 minute more.
Transfer the cooked onion and garlic to a large bowl. Add the minced beef, mustard, salt, pepper, and marjoram. Mix well until combined. Refrigerate the mixture for at least 30 minutes.
Shape the mixture into balls or cutlets.
Heat oil in a pan over medium heat. Fry the meatballs for 8–10 minutes, turning occasionally, until cooked through to an internal temperature of 71 °C / 160 °F.
Carefully pour the cooked meatballs into cleaned preserving jars. Place the jars in a pot of boiling water (100 °C / 212 °F) for 75 minutes.