Peel and finely dice the onions and garlic. Heat 4 tablespoons of oil in a saucepan. Brown the onions and garlic in it, dust with flour. Add tomatoes and vegetable stock, stir in tomato paste, season with salt, pepper and sugar. Simmer covered for about 10 minutes. Stir the cream into the sauce, season to taste again and let it cool down
Clean and wash the chard, cut white stems from the leaves. Cut leaves and stems separately into coarse strips. Heat 2-3 tablespoons of oil in a large pot. Steam the chard stems for 3-4 minutes, turning them over. Add the leaves, continue to steam for 3-4 minutes, season with salt and pepper, let cool. Dice mozzarella finely
Wash the fish, dab dry and cut into 1-1.5 cm wide strips. Grease an ovenproof dish (approx. 30 x 19 x 6 cm). Put some sauce in the dish, spread it out, cover with 4 lasagne plates. Put half the fish on top, add some sauce and cover with lasagne plates again. Place about half of the chard and some sauce on top, cover with about 1/3 mozzarella, cover with lasagne sheets.
Repeat the process, finishing with a layer of lasagne sheets and sauce. Sprinkle with the rest of the mozzarella. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for approx. 1 hour until golden brown