Before you start, preheat oven to 220°C (or 200°C fan bake).
Prep potatoes and spread in a single layer on a lined oven tray. Toss with olive oil and season with salt and pepper. Roast for 25-30 minutes, until tender and golden. Turn once during cooking.
Prep fennel and set aside in a medium bowl with rocket. Set aside some fennel fronds to garnish. Prep leek, butter and lemon. Pat fish dry, remove any remaining scales or bones and season with salt and pepper.
Heat oil in a medium fry-pan on medium-high heat. Cook leek and nduja for 5 minutes, stirring often, until leek is softened and coated in nduja. Add lemon juice and zest, white wine and water measure.
Bring sauce to a simmer and cook for 2-3 minutes, until reduced slightly. Turn heat to medium and whisk cold butter into sauce. Set aside and keep warm, until ready to serve.
Heat oil in a medium fry-pan on medium-high heat. Cook fish, skin side down, for 2 minutes, until golden. Flip and continue cooking for 1 minute or until cooked through. Rest, covered.
To finish, toss salad with olive oil and vinegar. Season with salt and pepper.
Serve potatoes topped with fish and sauce. Serve salad and reserved fennel fronds on top.