Preheat oven to 200°. Place potato in a large ovenproof pan and cover with cold salted water. Bring to the boil, then reduce heat to medium and cook for 15 minutes or until very tender . Drain and transfer to a bowl with butter and 2/3 cup milk. Mash until smooth. Season and set aside.
Add oil and leek to pan and return to medium/low heat. Cook, stirring, for 6-8 mins or until softened. Add bacon and cook for 5 minutes or until golden. Stir in garlic, cauliflower, thyme and trout. Season.
Place flour and fish in a bowl. Season and toss to coat. Add to cauliflower mixture . Stir in sour cream and reaming 2/3 cup milk, Top mixture with mash and smooth the surface with back of a spoon, swirling to create ridges. Transfer to the oven and bake for 50 minutes or until potato is golden. Set aside for 10 mins to rest, then season and sprinkle with dill. Serve with salad leaves and roasted lemon dressing.