Preheat the oven to 200°C.
Pulse flour and butter in a food processor until the mixture resembles breadcrumbs.
Blend in egg, water, and salt until the mixture forms a dough.
Roll out the dough on a floured surface and line a small tart tin.
Refrigerate the pastry for five minutes before trimming excess.
Blind bake the tart case with parchment and beans for ten minutes.
Sauté sliced onion in olive oil for four minutes.
Add garlic and mushrooms and cook for another four minutes.
Fill the pastry case with the mushroom mixture and top with Camembert, thyme, and cream.
Bake for 12 minutes until the pastry is golden and cheese melts.
Remove from tin and serve on a plate.