Cook the pasta in boiling salted water for about 10 minutes. Drain, rinse with cold water and let drain. In the meantime peel and finely chop the onion. Knead with minced meat, egg yolk and breadcrumbs.
Season with salt, pepper and paprika. Form small balls. Heat the oil in a pan and fry the meatballs in it over medium heat for 6-8 minutes and remove from the pan. Clean, wash and cut the leek into pieces.
Wash, clean and halve the mushrooms. Add the vegetables to the frying fat and fry for about 5 minutes. Season with salt and pepper. Deglaze with stock and cream and bring to the boil. Cook for 3-4 minutes at low heat.
Add the sauce thickener while stirring and bring to the boil again. Add pasta and meatballs and heat up briefly. Season to taste again. Clean, wash, quarter and seed the tomato. Cut into small cubes and sprinkle over the pasta pan.
Arrange on plates and serve garnished with parsley as desired.