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Bring a large pan of salted water to the boil. Tip in the potatoes and parsnips, bring back to the boil and simmer for 5 mins. Gently drain so they don’t rough up too much, then tip into a baking dish.
Melt the butter in a big saucepan, then stir in the flour and cook for 2 mins. Gradually stir in the milk to a smooth, lump-free sauce. Cook gently, stirring constantly, until the sauce is nicely thickened – about 5 mins. Stir in the mustard, honey, vinegar and some seasoning. Pour evenly over the potatoes. Mix the crumbs and cheese together, then scatter over the top and set aside until ready to bake. You can cover the dish and chill for up to 24 hrs before finishing.
Cook the bake at 200C/180C fan/ gas 6 for 30-40 mins until the top is crisp and golden and the sauce hot through.