Boil the potatoes in salted water as jacket potatoes, peel and cut into slices. Clean and wash the chard, blanch for 3 minutes (first put the stalks in the water), rinse immediately with cold water and allow to drain. Cut the fish fillet into pieces, drizzle with lemon juice, lightly salt and pepper. Grate the mountain cheese. Whisk the eggs with the crème fraîche and 2-3 tablespoons of water, mix in half of the cheese.
Spread 10 g butter on a heat-resistant dish, layer the potato slices and season with pepper. Spread half of the cheese cream on top and layer the sliced Swiss chard on top. Melt the remaining butter, pour over the vegetables, lightly salt and pepper. Spread the fish pieces on the vegetables, cover with the remaining cheese cream and sprinkle with the remaining grated cheese. Bake in a preheated oven at 200 ° C for 25-30 minutes.