Remove the crusts from the bread then place in a large bowl. Pour over the milk and allow to soak for a few minutes.
Add the beef, egg and seasoning and mix well.
With wet hands, form meatballs and set aside.
Heat a large, deep pan over medium-high heat. Add a splash of oil then add the meatballs and allow to brown until golden brown on all sides. Remove from the pan and set aside.
In the same pan, add the onion, garlic and mushrooms. Allow to cook until any liquid the mushrooms might have released has cooked off and the mushrooms start to brown.
Add the flour and stir in then pour in the stock. Stir until the sauce starts to simmer. Add the sour cream and mustard and stir until combined with the sauce.
Place the meatballs back in the sauce along with any of their resting juices. Bring to a simmer and cook for 7-10 minutes until the meatballs are fully cooked and the sauce is thick and creamy.
Season to taste and top with the parsley before serving.