First make the pastry. Sift the flour and salt into a bowl. Add the butter and rub in with the fingertips until the mixture resembles fine breadcrumbs. Add the egg yolk and measurement water and bring the mixture together. Wrap in clingfilm and chill for 30 minutes.
Roll the pastry out on a lightly floured work surface. Use to line a 25 cm (10 inch) fluted flan tin. Prick the base with a fork and chill for 30 minutes. Line the pastry with nonstick baking paper and baking beans and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 15 minutes. Remove the paper and beans and bake for a further 15 minutes. Leave to cool.
Meanwhile, melt the butter in a frying pan, add the shallots, garlic and thyme and cook over a low heat, stirring frequently, for 5 minutes. Increase the heat, add the mushrooms and salt and pepper and cook, stirring, for 4–5 minutes until browned. Leave to cool. Scatter over the tart case. Beat the soured cream, eggs, Parmesan and salt and pepper together and pour over the top. Bake for 20–25 minutes until golden and just set. Serve warm with some rocket leaves.