Cook the pasta in boiling salted water for about 8 minutes. In the meantime peel and chop the onion. Clean, wash and roughly chop the spinach. Clean, wash and slice the champignons. Wash fish and cut into small pieces.
Cook in 1/4 litre salt water and wine for about 5 minutes. Place the spinach on the fish, collapse and then drain, catching the stock. Sauté onion in hot fat. Sauté the mushrooms, turning them over, and season with salt and pepper.
Deglaze with the stock and cream, bring to the boil. Stir in the sauce thickener. Season to taste with salt, pepper and horseradish. Warm the fish and spinach in the sauce. Serve everything sprinkled with dill and garnished.