Step 1 Preheat oven to 200°C. Line a baking tray with foil. Place potato in a medium saucepan, cover with cold water and bring to the boil; cook for 12 minutes, or until tender. Remove potato from water, reserving water for cooking remaining vegetables.
Step 2 Peel potato and grate coarsely. Place fish on prepared tray, top with grated potato and spray with olive oil. Bake fish for 12 minutes, or until almost cooked through. (Cooking time will depend on fillet’s thickness.) Switch on oven grill; grill fish for 3–5 minutes, or until potato rosti is golden.
Step 3 Meanwhile, melt table spread in a small saucepan over medium– high heat. Add flour and cook, stirring, for 30 seconds, or until thick and bubbling. Add milk gradually, whisking until mixture is smooth; bring to the boil, then remove pan from heat and stir in parmesan to form a smooth sauce.
Step 4 Boil veg in reserved cooking water for 2 minutes, or until tender.
Step 5 Transfer rosti-topped fish to a serving plate, season with black pepper and serve with cheese sauce and vegetables.