In a medium saucepan cook the onion in the butter over medium-high heat until the onions soften, about 3-4 minutes. Remove from heat and let cool.
In a large bowl place the cooled onions and the rest of the meatball ingredients.
Using your hands, mix well for about 2 minutes until the ingredients are well combined.
Form small meatballs, set each one aside on a sheet pan or plate. You should have about 30 small meatballs.
To make the sauce: Heat 4 tablespoons of butter in a large sauce pan over medium heat. Add some of the meatballs. Work in batches if you have to, brown them on all sides. Use a slotted spoon and remove them from the pan, set them aside so you can make the sauce using the same saucepan.
Add 4 tablespoons of butter to the saucepan over medium heat.When the butter is warm, start stirring in flour with the wooden spoon until the mixture is thick.
Stir often with the wooden spoon over the heat, the flour will gradually begin to brown. Slowly add the beef stock a little at a time and keep stirring. Add the heavy cream, mustard, salt and pepper.
Add the meatballs to the sauce, cover the pot and turn the heat down to low.Cook for about 12 minutes.
Add the sour cream and mix well. Add the fresh parsley and serve over egg noodles.