Step 1 Cook penne in a large saucepan according to packet instructions; drain and return to pan. Cover pan to keep pasta warm; set aside.
Step 2 Meanwhile, heat table spread in a small saucepan set over low heat. Add flour and cook, stirring, for 1 minute, or until mixture starts to bubble. Whisk milk gradually into mixture until smooth; simmer for 4–5 minutes, or until sauce is thick and smooth. Stir half of the grated parmesan into sauce and remove pan from heat.
Step 3 Preheat oven to 180°C. Spray an 8-cup baking dish with olive oil. Spray a large non-stick frying pan with olive oil and set over medium heat. Add garlic and sliced leeks to pan; cook, stirring occasionally, for 8–10 minutes, or until tender. Stir baby spinach into mixture and toss over low heat until leaves wilt.
Step 4 Stir cheese sauce, leek mixture, tomatoes and dill into reserved pasta; add flaked salmon, toss and season with cracked black pepper. Spoon pasta mixture into prepared baking dish, scatter with remaining parmesan and transfer to oven; bake for 15–20 minutes, or until golden. Leave pasta bake to cool slightly, then transfer to an airtight container and freeze.
To reheat: Transfer pasta bake to fridge and leave to thaw for a day (or overnight). Place thawed bake in a shallow microwave-safe dish, cover and microwave on 50% power for 10–12 minutes, or until pasta bake is hot, and serve.