For the sauce, in a large saucepan, melt the butter and add the leek. Cook for 4-5 minutes with the lid on and stirring regularly until softened.
Add the flour and stir with a wooden spoon to make the roux.
Now, using a whisk, gradually add the milk.
Bring the sauce to boil and once thick and smooth, remove from the heat and season with salt and pepper, mustard, lemon zest, and capers.
Add half of the cheese and stir through before adding the spinach.
Preheat the grill element of the oven.
Bring a pot of salted water to the boil and cook the pasta until it’s al dente.
Use a slotted spoon to add the pasta to the sauce along with the salmon. Fold through.
Mix the remaining cheese with the breadcrumbs and oil.
Pour pasta into an oiled baking dish. Sprinkle over the crumbs and grill for 3-4 minutes or until golden brown.
