Peel and finely dice the onions and garlic. Heat 2 tablespoons of oil in a saucepan. Sauté a good 2/3 of the onions and garlic. Dust with flour and sauté briefly while stirring. Stir in the tomato paste and sauté briefly.
Add 1/4 l water while stirring continuously. Add the canned tomatoes and juice, crushing the tomatoes. Bring everything to the boil and simmer for about 5 minutes while stirring. Stir in cream. Season to taste with salt, pepper and about 1/2 teaspoon sugar.
Cover and simmer for about 10 minutes.
Clean and wash the chard and cut into wide strips. Heat 1 tablespoon of oil in a large pan and fry the rest of the onions, garlic and chard for 3-5 minutes. Season with salt and pepper.
Dice mozzarella. Wash the salmon fillet, dab dry and cut into 1-1.5 cm thick slices.
Preheat oven (electric cooker: 200°C/circulating air: 175°C/gas: level 3). Spread some tomato sauce on the bottom of a square casserole dish with a high rim. Place 4 lasagne plates next to each other on the sauce (if necessary, break them to size).
Layer half the fish, some sauce and 4 lasagne plates on top. Add the rest of the fish, then half the chard, 1/3 mozzarella and sauce.
Continue until all the ingredients are in layers, finishing with lasagne and sauce and sprinkle with the rest of the mozzarella. Bake in a hot oven for about 1 hour until golden brown. If necessary, cover with aluminium foil after 40 minutes.