Creamy chicken, chive and mustard gratin

Method

1. Preheat the oven to 200°C/fan180°C/ gas 6. Bring a saucepan of water to the boil and cook the potatoes for 6-8 minutes, until nearly cooked through. Drain well.
2. Make the sauce. Melt the butter in a saucepan, add the flour and cook for 2 minutes, until it forms a smooth paste. Add the stock, a little at a time, and simmer until you have a smooth, thickened sauce, stirring all the time.
3. Stir the crème fraîche and mustard into the sauce and heat for a few minutes. Add the chicken and chives and season to taste.
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5. Spoon the mixture into a 1.5-litre gratin or ovenproof baking dish and arrange the potatoes on top. Sprinkle over the cheese and breadcrumbs and bake for 20 minutes, until the top is pale golden and the filling bubbling hot. Preheat the grill to high and grill the gratin for 5 minutes to crisp up the topping. Scatter with the extra chives to serve.