Prepare 1.5 l vegetable stock and bring to the boil. Wash the quinoa with hot water and add to 1 liter of vegetable stock. Simmer for 15-20 minutes. Then drain and keep warm.
Squeeze the lemon. Drizzle the fish with lemon juice and let it steep for 10 minutes.
Blanch the spinach, then chop it roughly. Peel and finely chop an onion and sauté in butter in a pan. Add the spinach and heat. Fold in the crème fraîche, season with pepper, salt and nutmeg, then keep warm until serving.
Cut the dill very finely. Peel off the second onion and also chop very finely.
Heat the butter in a saucepan, sweat the onion in it and deglaze with the remaining vegetable stock, then bring to the boil and carefully stir in the mustard with a whisk. Add the cream and dill, then reduce the heat. Immediately before serving, stir in egg yolks over low heat, season with salt and pepper if necessary and serve.
Finally fry the pikeperch in a pan in butter for 2 - 3 minutes on each side, lightly pepper and salt and serve immediately.