Combine the ground meat with breadcrumbs, finely chopped onion, egg, mustard, and a pinch of salt and pepper in a large mixing bowl.

Chill the mixture in the refrigerator for about 10-15 minutes.

Shape the meat mixture into patties about ¾ inch thick.

Heat oil in a large frying pan over medium heat until shimmering.

Fry the patties for 4-5 minutes on one side, then flip and cook for another 4-5 minutes until golden brown.

Transfer the cooked Frikadellen to a paper towel-lined plate and serve hot.