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Heat oven to 200C/180C/gas 6. Toss the celeriac chips in 1 tbsp oil and tip them onto a large baking tray. Season and roast for 35 mins or until soft and golden around the edges. Brush the steaks with 1 tsp oil and sprinkle the lemon zest and some cracked black pepper liberally on both sides. Leave to marinate while you make the sauce.
Mix the parsley, capers, shallot, garlic, vinegar, hazelnuts and the remaining oil in a small bowl.
Heat a griddle or frying pan and cook the steaks for 2-3 mins each side if thick (1-2 mins for thin steaks). Rest, wrapped in foil, for 5 mins, then slice. Serve the steaks with the sauce and roasted celeriac.