Defrost the spinach leaves and squeeze them well. Wash potatoes, cook in boiling salted water for about 20 minutes, peel and slice. Peel onions and cut into strips. Heat oil, fry onions in it until transparent.
Add the spinach. Season with salt, pepper and nutmeg and deglaze with vegetable stock. Rinse fish fillets, dab dry, sprinkle with lemon juice and season with salt. Grease a large casserole dish or 4 portion dishes.
Layer the potato slices in flakes. Spread spinach on the potatoes. Place the fish fillet in the form. Wash parsley and thyme, dab dry and chop. Remove the crusts from toast, dice coarsely and grind finely in a lightning chipper.
Grate parmesan. Melt butter, mix with toast, parmesan and herbs and pour over the fish. Gratinate in the preheated oven (electric cooker: 200 °C/ convection oven 175 °C/ gas: level 3) for about 15 minutes.
Garnish with parsley and lemon slices.