Pre-heat the oven to 200°C. Roll out the pastry on a lightly floured surface to 1cm thick and cut rounds of pastry slightly larger than the moulds of the fluted mini tart tins. Line fluted mini tart tins using the pastry, pressing in well and prick the bases. Trim any overhanging pastry and chill for 10 minutes. Very quickly saute the mushrooms in the olive oil in a large frying pan set over a moderate heat. Remove from the heat and set to one side. Line the pastry cases with nonstick baking paper and fill with baking beans. Bake in the oven on a baking sheet for 10-12 minutes until golden. Remove the paper and beans at this point, brush the bases with the beaten egg yolk and return to the oven to allow the base to cook and brown. Meanwhile, make the filling by whisking the crème fraîche and the milk together. Beat in the eggs one at a time and season well with salt and pepper. Remove the pastry cases and pour the filling into each one so that it comes three quarters of the way up. Arrange the mushrooms and walnuts in each one and return to the oven on a baking sheet. Bake at 170°C for 15 minutes until the filling has just set. Remove from the oven and allow to cool for a few minutes on a wire rack before turning out onto serving plates. Garnish with sprigs of chervil before serving.