To make the crêpes, sift the flour into a mixing bowl and stir in the salt and chives, make a well in the centre and add the egg, gradually add the milk and whisk until you have a smooth creamy batter. Cover and leave to stand for 30 minutes. To make the filling, melt the butter in a small pan and stir in the flour, gradually add the milk and cook stirring until you have a smooth thickened sauce. Stir in two thirds of the grated red Leicester and mustard.
Heat the olive oil in a pan and gently fry the leeks and mushrooms until softened. Pour two thirds of the sauce over and mix well. To cook the crepes, heat a 20cm (8in) non stick frying pan and wipe with a little of the oil. Whisk the batter again and using a ladle, swirl a thin layer of the batter over the pan – just enough to give a thin coating. Cook for 2 minutes then flip over with a palette knife and cook the other side for a further 2 minutes. Stack onto a warmed serving dish and keep warm whilst you cook up the remaining batter - you should be able to make 8 crepes. Wipe out the pan with a little extra oil before you cook each crepe.
Preheat the oven to Gas Mark 4, 180°C, fan 160°C. Spoon a little of the filling down the centre of each crepe and roll up, lift into an ovenproof dish. Drizzle the remaining sauce over the crepes and sprinkle with remaining cheese. Bake for 20 minutes. Serve hot - with a rocket and cherry tomato salad.
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