First make the pastry. Mix together the flour and salt then add the mustard and cold butter. Use your fingertips to rub it all together until you have a sandy texture. Add the vodka to bring it into a dough.
Place between two sheets of baking paper and roll into a circle that is larger than your quiche tin. Leave to chill in the fridge for 30 minutes. Preheat the oven to 180 C, 160 Fan, Gas 4.
Remove the pastry from the fridge and line the quiche tin. If it is too hard let it soften for 5-10 minutes before rolling. Prick the pastry in the tin with a fork. Line with baking paper and pour in baking beans. Bake for 20 minutes then remove the paper and baking beans and bake for 10 minutes more, or until the base is dry and firm.
To make the filling beat the eggs with the cream, salt and pepper until you have a smooth mixture. Stir in the cheese. Pour into the pie case, put the onions on top and bake for 25-30 minutes or until set but still a little wobbly.
When the pie has cooled heat the butter for the mushrooms in a large frying pan. When it is foaming add the mushrooms and cook for 4-5 minutes or until lightly golden and cooked through. Leave to cool slightly.
Scatter the pie with parsley and top with the mushrooms to serve.