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 4 hours, ideally overnight.
Shape the set mixture into cylinders about 8-10 cm long and 3 cm in diameter (this amount should yield approx. 10-12 croquettes).
Prepare three bowls: one with flour, one with beaten eggs, and one with breadcrumbs.
Coat the cylinders thoroughly – first in flour, then in egg, and finally in breadcrumbs. Double Coating: Dip them again in the egg and then back into the breadcrumbs. This double coating is critical to prevent the croquette from bursting during frying.
Heat the oil in a deep fryer or deep pot to 185 °C (365 °F).
Deep-fry the Kroketten in small batches for 4-6 minutes until beautifully dark golden brown. The inside must be hot and liquid.
Remove, drain on paper towels, and serve immediately extremely hot, ideally with bread and mustard.