Melt the butter in a saucepan over medium heat, then stir in the finely chopped onion and sauté until soft and translucent, about 3 minutes.

Add the 1/2 cup of flour to the butter and stir constantly for 2 minutes to create a roux, cooking until it smells nutty but hasn't browned too deeply.

Gradually whisk in the beef stock a little at a time, stirring vigorously to ensure a smooth, thick sauce forms without lumps.

Simmer the sauce for a few minutes until thickened, then stir in the shredded beef, chopped parsley, nutmeg, salt, and pepper.

Transfer the mixture to a shallow container, cover with plastic wrap touching the surface to prevent a skin forming, and refrigerate for at least 3 hours (or overnight) until the ragout is completely solid.

Once chilled, take a tablespoon of the mixture and roll it between your palms to form a smooth ball, repeating until all the mixture is used.

Set up a breading station with three bowls: one with plain flour, one with beaten eggs, and one with breadcrumbs.

Roll each ball first in the flour, then dip into the beaten egg, and finally coat thoroughly in the breadcrumbs.

Heat the vegetable oil in a deep fryer or heavy pot to 375°F (190°C).

Fry the bitterballen in batches for about 3-4 minutes until they are golden brown and crispy on the outside.

Remove with a slotted spoon and drain on paper towels to remove excess oil.

Serve immediately while hot with a side of spicy or grainy mustard for dipping.