Melt the butter in a sauce pan and let it slowly take a nut brown colour while turning all the time. Keep it on medium heat in order not to burn the butter
Add the flour and keep cooking while constantly whisking until the mixture has reached a brown color, watch the heat to avoid burning
Add the beef stock little by little while whisking vigorously
Add whatever taste enhancement you want or have; red wine, soya sauce, salt, pepper, lingonberry jam, cream... I add it all :-)
Let it simmer for a few minutes and then add the meatballs to the sauce. Let it simmer at low heat until the meatballs are cooked through. Avoid stirring too much once the meatballs are in the sauce to avoid the sauce getting troubled by bits falling off from the meatballs