In a large saute pan, melt the butter over medium heat. Once melted, slowly stir in the flour to make a paste. Cook for a minute, while stirring, then slowly mix in the stock. Reduce heat to medium low and continue to cook, stirring, until the mixture has thickened. Stir in the onion, parsley, and beef. Season with nutmeg, salt, and pepper.
Transfer the mixture to an airtight container and refrigerate overnight.
Once the mixture is thoroughly chilled and thickened, fill a pot or deep fryer with 3-4 inches of vegetable oil. Heat the oil to 375˚F. Cover a plate with a towel and place nearby.
Add the flour to a shallow bowl. In another bowl, beat together the eggs. In a third bowl, add the bread crumbs. Place a plate nearby.
Remove a tablespoon of the thickened filling and roll it into a ball. Coat in the flour, thoroughly in the egg, then in the breadcrumbs. Place on the plate and repeat with another 4-5 pieces. Add to the hot oil and fry until golden. While they are frying, prepare a few more balls. Remove the golden bitterballen to the towel lined plate. Repeat with the remaining mixture.
Serve hot with mustard.