To Make the Pastry:

1. This recipe makes two tarts.
2. In a mixing bowl, combine the flour with a pinch of salt, then rub in the butter with your fingertips. You'll get a mixture that looks like bread crumbs. To this, add some cold water, a tablespoon at a time, mixing and pressing the dough together with your hands as you do. You will not need all the water, just enough to form a smooth, pliable dough.
3. Tip the pastry out onto a clean, floured surface and roll out with a rolling pin to a thin, even sheet. Press carefully into the tart dishes, trim, and refrigerate while you make the filling.

To Make the Filling:

1. Preheat the oven to 435°F. Heat up the olive oil in a large frying pan. Over a medium-high heat fry the garlic and onion with the sugar until golden and caramelized, stirring frequently.
2. Add the capers and mushrooms and continue frying. Once the mushrooms are fully cooked, add the cream and the thyme leaves (reserving a few to top the tarts), and cook for a further 5 minutes or so. Sprinkle in a tablespoon of flour and stir till thickened. Season to taste and stir again. Take off the heat and remove your pastry cases from the refrigerator.

To Make the Tarts:

1. Spoon the hot mushroom filling into the tart cases. Bake for 10 minutes, then another 20 minutes or so at 390°F, or until the tarts are caramelized on top.
2. Top the tarts with more thyme and serve hot or leave to cool and refrigerate.