Begin by melting the butter gently in a skillet or heavy-bottomed pan over medium heat, ensuring it doesn't brown but melts smoothly. Once melted, gradually add the flour in small portions, stirring continuously to create a smooth, thick paste known as a roux. This base is crucial for the creamy filling so take your time to avoid lumps. Next, slowly pour in the beef stock while stirring vigorously to help the roux absorb the liquid evenly. Reduce the heat to low and let the mixture simmer gently for a few minutes, stirring occasionally to prevent sticking. Then, fold in the finely chopped onion, freshly chopped parsley, and the shredded cooked beef. Season with a pinch of salt, pepper, and nutmeg to enhance the savory depth. Continue stirring as the mixture thickens into a luxurious, heavy sauce. Transfer this rich beef mixture into a shallow dish and cover it tightly with plastic wrap. Place it in the refrigerator for several hours or until it has fully chilled and hardened; this step is essential to make shaping the bitterballen easier. Once chilled, scoop generous tablespoons of the firm filling and swiftly roll them into smooth, round balls using your hands. To prepare each bitterbal for frying, lightly dust each ball with flour, then dip it entirely into beaten eggs, and finally coat it thoroughly with breadcrumbs. Be sure the egg wash covers every surface so the breadcrumbs adhere well and fry evenly. Place the breaded bitterballen back into the refrigerator to chill once more, which helps them hold their shape when fried. Heat your oil in a deep fryer or heavy pot to 190°C (375°F). Fry no more than four at a time to avoid temperature drops, and cook until they achieve a gorgeous golden-brown color—usually a few minutes. Remove and drain on paper towels to get rid of excess oil. Serve these delightful Dutch meatballs hot, accompanied by a plate of grainy or spicy mustard for dipping. For the best experience, enjoy them fresh and crispy as a delicious snack or appetizer at your next gathering.