Melt the butter in a large pot and add the onions. Let cook for 10 minutes and add the 2 cups red wine. Continue cooking until almost all the liquid evaporated. Season with sugar and salt and pepper to taste. Place the salmon filet in baking dish and add the remaining red wine and cream. Sprinkle with salt and pepper. Bake at 250ºC or 7 minutes. Lift out the cooked salmon and set aside. Pour the liquid into a pot and reduce until syrup consistency. Adjust seasoning. Place some onion marmalade on top of salmon and spoon some sauce over it and serve