Clean and wash the spinach, chop it coarsely and drain well. Peel and finely chop the onions. Melt 1 tablespoon butter. Fry a diced onion in it. Add the spinach and let it collapse.
Season with salt and pepper, let cool on a sieve, press out again if necessary. Grate or slice Parmesan cheese. Mix quark, half of the Parmesan, eggs and egg yolk until smooth. Mix in the spinach. Season again with salt and pepper.
Knead in the flour little by little. Chill for 2-3 hours. Form 12 dumplings with lightly floured hands and let them simmer in lightly boiling salted water until they float to the surface.
For the sauce, clean and halve the mushrooms. Melt 1 tablespoon butter. Fry the remaining onion cubes and mushrooms for about 5 minutes. Season with salt and pepper. Deglaze with 250 ml water and cream, bring to the boil.
Break the cheese into pieces, stir into it, season again. Chop the walnuts into flakes. Melt another tablespoon of butter. Fry the nuts and dumplings for 2-3 minutes. Sprinkle with the sauce and the rest of the Parmesan cheese.