As the filling is a little too gooey for slicing, it is better if you use 4 tart tins measuring approximately 2½ – 3cm high by 10cm. Cut the pastry about 2cm larger than the tins and line them, firming the pastry up to and a little over the top of the tins with your thumb and forefinger. With a fork, prick the pastry all over on the inside of the tins, including the sides. Line them with greaseproof paper or foil and fill them with baking beans (dried old beans or the metal baking beans you can buy).
Leave to rest in the fridge for 1 hour at least or better still overnight. Pre-heat the oven to 180C/gas mark 4. Bake the cases for 20 minutes, then remove the baking beans and return to the oven for 5-10 minutes until lightly coloured. Remove pastry from the tins and leave on a wire rack to cool.
Gently cook the leek in the butter for 2-3 minutes until soft, add the mushrooms, season and cook with a lid on for 3-4 minutes, stirring every so often until they soften. Add the cream and cheddar and continue to simmer until the cream thickens and just coats the mushrooms, parsley and re-season if necessary.
To serve, fill the tarts with the mushroom mixture and reheat for 5-6 minutes then make an indentation in the centre of the filling and crack a quail egg in the centre of each tart and return to the oven until the egg is just cooked.