Cook the soup meat (cubed beef) in the beef stock with 1 teaspoon salt for about 2-3 hours, until it’s fork-tender. Let it cool to room temperature and finely slice the beef. Take 2 cups (500 ml) of stock and set aside.
Slice the onion finely.
Make a roux by melting the butter and glazing (browning) the onion for about 2 minutes.
Then add the flour all at once and stir on heat for another 3 minutes.
Add the stock, whisk well and let it come to a boil (keep stirring). The roux absorbs all the liquid. Cook at a high simmer for another 5 minutes. Add the cooked soup meat (cubed beef).
Add the nutmeg, pepper, and salt to the beef mixture.
Take a cooled plate or oven dish (rinsed in cold water) spread the filling over the plate and cover with some plastic wrap and allow to cool down for at least 2 hours in the refrigerator (better to let it cool overnight).
Put three bowls in front of you and fill the first one with flour, the second one with the eggs (beat the eggs), and the third one with bread crumbs.
Take the filling and take a spoon of filling and make a ball of it. Roll it through the bowl with flour, then the eggs, and at last through the bread crumbs.
Repeat the breading (crumbing) process again if you wish.
Place on plate and repeat for the rest of the filling. Cover and refrigerate for 30 minutes.
Heat your frying pan to 350°F/180°C (medium heat). Check the temperature by placing a small cube of bread in the heated oil it should sizzle and brown in a minute or two.
Deep fry the bitterballen for about 3 to 4 minutes until done (golden brown).