In a large saucepan, melt 4 tablespoons butter over medium heat. Add finely chopped onions and minced garlic, sauté until softened, about 3-4 minutes.

Add the ground beef and cook, breaking it apart with a wooden spoon, until browned and fully cooked, about 8-10 minutes. Season with salt, black pepper, nutmeg, and a bay leaf.

Sprinkle flour over the meat mixture and stir constantly for 2 minutes to cook out the raw flour taste.

Gradually pour in the beef stock while stirring continuously to prevent lumps. Add worcestershire sauce and continue stirring until the mixture thickens into a very thick, paste-like consistency, about 5-7 minutes.

Remove from heat, discard the bay leaf, and stir in fresh parsley. Transfer the mixture to a shallow dish, spread evenly, cover with plastic wrap touching the surface, and refrigerate for at least 4 hours or overnight until completely firm.

Once chilled, use a small scoop or spoon to portion the mixture into 24 equal balls, rolling each between your palms to create smooth spheres. Place on a parchment-lined tray.

Set up a breading station with three shallow bowls: one with flour, one with beaten eggs, and one with breadcrumbs. Roll each ball first in flour, then egg, then breadcrumbs, ensuring complete coverage. For extra crunch, repeat the egg and breadcrumb coating.

Heat vegetable oil in a deep fryer or heavy pot to 350°F (175°C). Fry the bitterballen in batches of 6-8 for 3-4 minutes until deep golden brown, turning occasionally for even cooking.

Remove with a slotted spoon and drain on paper towels. Let rest for 2-3 minutes before serving to avoid burning your mouth on the hot filling.

Serve immediately with spicy mustard and cold beer.