Step 1 Preheat oven to 200°C. Heat olive oil in a large non-stick frying pan set over medium–high heat. Add onion; cook for 2–3 minutes. Add mushrooms and asparagus to pan; cook, stirring, for 3 more minutes, or until tender. Remove pan from heat and set aside.
Step 2 Set a small saucepan over low heat. Melt table spread and add flour and pepper; cook, stirring, for 1 minute, or until bubbling. Increase heat to medium and pour milk into pan in a gradual stream, stirring to prevent lumps; simmer, stirring, until sauce is smooth and slightly thick. Add half of the grated parmesan to pan, stir and set aside.
Step 3 Spray a large baking dish with olive oil. Add reserved veg, pasta, fish and a little of the reserved white sauce to dish; stir gently.
Step 4 Pour remaining white sauce over fish mixture; scatter with remaining parmesan. Bake for 20–25 minutes, or until fish cooks through and cheese turns golden.
Step 5 Serve gratin with salad greens.