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Heat oven to 200C/180C fan/gas 6. Toss all the vegetables with the oil and season well. Spread in a single layer on 2 baking trays (or 1 very large tray) and roast for 30 mins.
Season the salmon and spread over the mustard. In the final 10 mins of cooking the veg, add the salmon to the trays.
In a small bowl, mix together the horseradish, crème fraîche, vinegar, dill and some seasoning. Serve the salmon with the sauce and veg.