Sift flour and salt into a mixing bowl. Meanwhile melt butter with the water over low heat and pour into bowl and mix to a soft dough. Press evenly into a 18-20cm/7-8 inch cake tin and working the dough about an inch up the sides.
Melt butter, add mushrooms and cook for about five minutes. Remove the mushrooms and stir flour into the butter, slowly add the milk and bring to the boil, stirring continuously. When thickened, remove from the heat and add the cheese mustard and seasoning.
Soften the curd cheese in a bowl and mix in the cheese sauce, egg yolks and sour cream. Whisk egg whites until stiff and fold in together with the cooked mushrooms. Spoon into the tin and level. Cook for 1hr 30 mins at Gas Mark 3/160C
For the topping, cook the mushrooms in butter until tender and arrange on the top of the cheesecake. Sprinkle with the cheese and return to oven for about 15 mins.
Ease gently out of tin and serve warm.
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