Thaw the spinach in a little water and half of the cream in a saucepan over low heat, so that a liquid sauce is formed. Season with salt and pepper.
For the sauce, melt the margarine, add the flour and brown briefly. Deglaze with the hot milk. Add the cream and water (amount as required). Season with plenty of tomato paste, garlic and onion salt and pepper.
Cut the salmon into small cubes while half-frozen.
In a medium baking dish, layer as follows: sauce, lasagne sheets, spinach, lasagne sheets, sauce and diced salmon, lasagne sheets, sauce and grated cheese.