Wash the chard, drain well and cut out the white stalks. Peel and finely chop the onion and clove of garlic. Peel and grate the ginger.
Melt the butter in a saucepan and fry the onion with the garlic until translucent. Add the chard and cook everything covered for approx. 5 minutes. Season with salt, pepper, nutmeg and ginger. Rinse the fish with cold water, pat dry, season with salt, pepper and drizzle with the lime juice.
Preheat the oven to 180 ° (fan oven 150 °) and grease a flat gratin dish. Cover the bottom with half of the Swiss chard, place the fish fillets on top and cover with the remaining Swiss chard.
Mix the cream, egg and cheese together. Season with salt, pepper and nutmeg and pour over the gratin. Cook on the middle rack for approx. 30 minutes.