Preheat the oven to 180°C/350°F/Gas 4. Line a baking tray with parchment paper.
For the savory pastry cases, sift the flour into a large bowl and, using your fingertips, rub in the butter until the mixture resembles fine breadcrumbs.
Stir in the salt and then mix in enough of the egg yolks to give a soft dough, adding a little cold water if you run out of egg yolk.
Wrap the pastry in cling film and leave it to rest in the fridge for 20-30 minutes.
Roll the pastry out thinly on a lightly floured work surface and use it to line 4 tart tins, 10cm in diameter.
Prick the base of the pastry all over with a fork. Line the each pastry case with a piece of greaseproof paper and place a layer of dried beans, lentils or pulses on top of the paper.
Bake the cases in the oven for 10 minutes, remove greaseproof paper and dried pulses and cook for a further 5-10 minutes until they are golden brown.
Take out of oven and allow them to cool before removing them from the tins.
For the filling, carefully clean the mushrooms, cutting the larger ones in half and gently fry them in a pan with the oil, herbs and garlic until they are tender.
Season the mushrooms with salt and pepper, remove them from the pan and set them aside to keep warm.
Return the pan where the mushrooms were cooked to the heat and add the leeks.
Cook them over a moderate heat until they are soft and season them well.
Meanwhile, warm the pastry cases and the onions through in a warm oven and fill the tarts with the mushroom mixture.
Arrange some of the warm leeks in the centre of each of the 4 serving plates.
Place the mushroom-filled tarts on top, with the parmesan cheese scattered over them and some chives across the top.
Arrange the warmed onions around the edge of the plate and serve immediately.