Melt the butter in a saucepan. Add the flour and stir for 2 minutes until a light roux forms.
Gradually pour in the warm broth, stirring constantly until a very thick béchamel forms. Bring to a boil and cook for 1 minute until the mixture pulls away from the bottom of the pan.
Remove from heat, add the shredded beef, parsley, salt, pepper, and nutmeg. Mix well.
Transfer the mixture to a shallow dish, cover with film (directly on the surface to prevent skin formation), and let it set in the refrigerator for at least 3 hours, ideally overnight.
Shape the set mixture into small balls about 3-4 cm in diameter (about 15-20 grams).
Prepare three bowls: one with flour, one with beaten eggs, and one with breadcrumbs.
Thoroughly coat the balls – first in flour, then in egg, and finally in breadcrumbs. For extra crispiness, repeat the coating in egg and breadcrumbs (double coat). This is important to prevent the filling from leaking out.
Heat the oil in a deep fryer or deep pot to 185 °C (365 °F).
Deep-fry the Bitterballen in small batches for 3-5 minutes until beautifully golden brown. The inside must be hot and liquid, but not boiling, otherwise they will burst.
Remove, drain on paper towels, and serve immediately extremely hot with a bowl of mustard.