Wash spinach thoroughly, clean and spin dry. Blanch in plenty of boiling salted water for about 1 minute, remove and rinse with cold water. Drain the spinach, squeeze out well and chop finely.
Wash the lemon hot, grate dry and finely grate the peel. Mix half with spinach, egg and double cream. Season well with salt and pepper.
Spread the spinach in a large roaster (approx. 25 x 30 cm). Rinse fillets cold and dab dry. Place them side by side on the spinach and season with salt, pepper and 2-3 teaspoons lemon juice. In a preheated oven (electric cooker: 175 °C/ circulating air: 150 °C/ gas: see
manufacturer) cook for 15-20 minutes. Remove from the oven and sprinkle with the remaining lemon zest.