Begin by making the shortcrust pastry. Pulse the flour, butter, and salt in a food processor, until it reaches the consistency of breadcrumbs. Steadily add the water, stopping as soon as the dough comes together. Remove the dough, knead it a couple of times, then wrap and allow it to chill in the fridge for at least 30 minutes.
Heat the oven to 180C/160C (fan), then roll the chilled dough into a thin circle, large enough to line a 24cm, loose-bottomed tart tin, ensuring extra pastry forms an overhang.
Prick the base, line the pastry with baking parchment and baking beans, then blind bake in the oven for 20 minutes.
Remove the parchment and beans and return to the oven for a further 5 minutes, or until the base is dry and lightly golden. You can trim away any overhanging pastry or leave it for a more rustic finish.
To make the filling, preheat the oven to 180C/160C (fan).
Pack the dried porcini into a small bowl, pour over just enough boiling water to cover them and let the mushrooms rehydrate. Set a large, heavy-based frying pan over a mediumhigh heat, add the oil and when hot, add the bacon. Fry the bacon, stirring regularly until it becomes crisp around the edges. Add the onions to the bacon and fry for 6-8 minutes.
Lift the bacon and onions out into a bowl and return the pan to the heat. Carefully scoop the rehydrated mushrooms from their bowl, reserving the liquid, then add to the pan along with the sliced chestnut mushrooms. Season with salt and pepper and cook for 4-5 minutes. Add half the thyme leaves and the chopped parsley, then tumble the mushrooms together with the bacon and onions.
In a large mixing bowl, beat the organic eggs with the cream and around 150ml of the liquid from the soaked mushrooms, then season with salt and pepper.
Fill the tart case with the mixture, but don’t press it down, and pour over the creamy egg mixture. Pat the fillings down into the egg mixture and scatter over the remaining thyme.
Place the tart in the oven and cook for 30-35 mins, or until it has a mottled, golden top and is slightly raised.
Allow to rest for 20 minutes, before diving in.