Season salmon with juice of one lemon, salt and pepper, infuse a little bit. Remove the skin from the onion and cut into small cubes. Heat a little oil in a large frying pan and sauté the onion. Add the cleaned spinach and sauté until soft, season with salt and freshly ground pepper. Rinse the long grain rice, put it in boiling salted water, simmer on low heat for about 20 min, stir in the saffron at the end. In a saucepan, cream the butter, add the flour and pour in the milk and water. Stir frequently to avoid lumps. Simmer this béchamel sauce a little on low heat. Grate the cheese and stir it into the béchamel sauce. Butter a shallow gratin dish, place long-grain rice in it evenly. Place the salmon fillets on the long grain rice, top with spinach leaves and the king prawns cut into pieces, pour the cheese sauce over them. Bake in the oven heated to 200 degrees Celsius for 20 minutes. Serve the salmon fillet au gratin and saffron rice on plates or in the dish.
Tip: Did you know that one bowl of spinach provides almost all the manganese and fiber you need?