Wash the redfish fillet and dab dry, if necessary cut it a little smaller. Sprinkle with lemon juice, salt a little. Cover and let it stand. Sort the spinach and wash thoroughly. Peel onion and garlic and chop finely.
Heat 20 g fat. Sauté onion and garlic in it until transparent. Add spinach, sauté and let it collapse. Season with salt, pepper and nutmeg. Melt the remaining fat. Sauté flour in it and deglaze with stock and 150 g cream.
Bring to the boil while stirring. Season with salt, pepper, horseradish and tarragon. Whisk the egg yolks and the rest of the cream, then stir into the sauce. Do not let it boil anymore. Layer the lasagne plates, some sauce, fish fillet, lasagne plates, sauce, spinach, lasagne plates and remaining sauce in a greased casserole dish.
Sprinkle with parmesan. Bake in the preheated oven (electric oven: 200 °C/ gas: level 3) for 40-45 minutes. Serve garnished with lemon wedges and dill.