Clean the mushrooms, reserve half and finely chop the rest in a blender. Add 1 tsp salt and leave to stand for 1 hour.
Meanwhile, coarsely dice the rolls and soak in 150ml warm water. Squeeze them well and place in a large bowl.
Peel and dice the onion and add to the soaked bread.
Drain the chopped mushrooms in a sieve, collecting the mushroom juices.
Add the mushrooms, beef mince and egg to the bread mixture. Season with salt and pepper, then mix with a mixer fitted with dough hooks to a firm mixture.
Shape into an oval.
Heat the oil in a large pan and fry the meatball over a medium heat until browned on all sides.
Mix the tomato juice with the mushroom juice and stock, pour over the meatball, and cook, covered, over a medium heat for about 30 minutes, turning from time to time.
Meanwhile, wash the potatoes and cook in lightly salted boiling water for 20 minutes. Cut the remaining button mushrooms into thin slices.
Remove the meatball from the pan, wrap it in foil and leave to rest for about 5 minutes.
Place the mushrooms in the cooking liquid and bring to the boil.
Stir the flour with the cream until smooth, and add to the sauce. Simmer for 5 minutes over a low heat until the sauce thickens. Season with salt and pepper.
Drain the potatoes, rinse in cold water and peel. Serve with the meatloaf and the sauce.