1- Preheat the oven to 190C (375F).
2- In a saucepan, melt the butter. Add the leeks and sauté for 2 minutes. Add the salmon and cook, over medium heat, until the salmon is lightly browned and the leeks are softened.
3- Transfer the salmon and leeks to a baking dish.
4- Cover with sweetcorn and mashed potatoes. Sprinkle with grated cheese and bake for about 25 minutes.
5- Serve very hot with a green salad.