Wash, peel and dice the carrots. Cook in boiling salted water over medium-high heat for 8 minutes until tender. Drain and set aside. Chop the mushrooms and pickled cucumber into small pieces.
Squeeze the excess water from the soaked roll. Mix the minced meat with the squeezed roll, egg and mustard in a bowl. Season with salt, pepper and paprika powder. Form the mixture into balls.
Cut the onions into wedges. Heat 2 tablespoons oil in a large pan over medium-high heat. Fry the onions for 3-4 minutes until softened. Remove and set aside.
In the same pan over medium-high heat, fry the meatballs on all sides for 5-6 minutes until browned. Remove and set aside, keeping warm.
In the same pan, fry the mushrooms for 2-3 minutes. Add the onions and carrots. Pour in the vegetable stock and cucumber liquid. Stir in the cream and sauce thickener. Add the pickled cucumber pieces. Bring to a boil over medium heat.
Season the sauce with salt, pepper, paprika powder and a pinch of sugar.
Return the meatballs to the pan and bring to a boil for 1-2 minutes. Serve.