Preheat the oven to 200C/400F/Gas 6.
For the pastry, place the flour and butter into a food processor and pulse until the mixture resembles breadcrumbs.
Crack in the egg and add the water and salt and blend again until the mixture comes together as a dough.
Turn out onto a floured surface and roll out to the thickness of a pound coin. Line a small tart tin with the pastry and trim away the excess. Refrigerate for five minutes.
Line the tart with baking parchment and fill with baking beans. Transfer to the oven and bake for ten minutes. Remove the baking beans and parchment.
For the filling, heat the olive oil in a frying pan, add the onion and gently fry for 3-4 minutes.
Add the garlic and mushrooms and cook for 3-4 minutes.
Spoon the mushroom mixture into the pastry case. Top with the Camembert, thyme and cream.
Transfer to the oven and bake for 10-12 minutes, or until the pastry is golden-brown and the cheese is beginning to melt.
To serve, remove from the tart tin and place onto a serving plate.