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Boil the sweet potatoes and parsnips for 20 mins in salted water until tender. Drain really well, then roughly mash with half the crème fraîche and half the sage, the mustard and salt and pepper. Pile into a buttered ovenproof dish and smooth over.
Heat oven to 190C/170C fan/gas 5. Mix the breadcrumbs with the Parmesan, pine nuts and remaining sage. Spoon the remaining crème fraîche over the mash, scatter over the bread and cheese mixture, then bake for 35-40 mins from fridge-cold, 25-30 mins if not, until golden.
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