Start this recipe by ½ cooking the pasta shells. Meanwhile, make the cheese sauce by putting the milk, flour and butter in a medium sized saucepan and heating gently. As the milk starts to warm up, take a balloon whisk and gently start whisking the sauce. When the milk boils, the sauce will begin to thicken and become glossy. Turn down the heat, and continue to cook, just for a couple of minutes. Turn off the heat, add 150g of the grated cheddar cheese, a pinch of cayenne pepper and nutmeg, season to taste with salt and pepper and mix well. Next, you will need to cut the broccoli into small florets and blanch them in hot water for a few minutes, keeping them green and crunchy. You will also need to poach the salmon too, again only for a few minutes. When the salmon is ready, remove the skin and any bones and discard. Gently flake the flesh with a fork. In a large bowl, put in the cooked pasta, broccoli florets, poached salmon and cheese sauce and gently mix together. Pour the mixture into a large oven proof dish and sprinkle the remaining 50g of grated cheddar cheese. Finally, sprinkle the bread crumbs over top. Bake in the oven for just 15 minutes, or till the sauce is bubbling and the cheese on top is golden brown.