Once ready to serve, fill a pot or deep fryer with 3-4 inches (8-10 centimeters) of vegetable oil and heat the oil to 375˚F (190˚C).
Place a large plate nearby and cover with a towel.
Place the flour in a wide, shallow bowl.
In another bowl, beat together the eggs.
In a third bowl, add the bread crumbs.
Place another plate nearby.
Scoop out a tablespoon of the thickened filling and form into a ball. Coat in the flour, then thoroughly in the egg, allowing any excess to drip off. Finish by completely coating in the breadcrumbs. Place on the plate and repeat with another 4-5 pieces.
Gently add the formed Bitterballen to the heated oil and fry, turning occasionally, until golden and heated through.
Use a slotted spoon to remove the golden Bitterballen to the towel-lined plate.
Repeat with the remaining beef mixture, coating in the flour, eggs, and breadcrumbs, then frying until golden.
Serve the Bitterballen while still warm with mustard for dipping.