Cook the pasta in a large saucepan of boiling water following packet directions or until al dente. Drain well.
Heat a large non-stick frying pan over medium heat. Cook salmon for 2 mins each side or until browned and almost cooked through. Transfer to a plate.
Add leek to the pan and cook, stirring, for 5 mins or until softened.
Melt the butter in a large saucepan over medium heat. Add the flour and mustard and cook, stirring, for 2 mins or until mixture bubbles and is grainy. Reduce heat to low. Gradually add milk, stirring constantly. Cook, stirring, for 5 mins or until sauce boils and thickens. Remove from heat. Add the cream and cheddar. Stir to combine. Season.
Add pasta to sauce with leek, peas, spinach and dill. Coarsely flake salmon. Add to pasta mixture. Gently toss to combine. Transfer to a 12-cup (3L) ovenproof dish. Top with breadcrumbs.
Preheat oven to 180°C. Bake for 30 mins or until golden brown and heated through.
