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Heat the oven to 200C/180C fan/gas 6. Toss the parsnips with half the olive oil and some seasoning. Roast for 20 mins, drizzle with the honey, then roast for 10 mins more or until the parsnips are golden, sticky and tender. Meanwhile, make a dressing. Mix together the lemon juice, the horseradish and the remaining oil in a large bowl.
Toss the parsnips, watercress and lentils into the dressing, then divide between four plates. Top with the smoked mackerel and serve with the lemon wedges.