1. Preheat the oven to 400°F (200°C). Put the mushrooms, olive oil, lemon thyme, and salt and pepper in a bowl and mix together. In another bowl, stir together the tapenade and créme fraîche.
2. Unroll the pastry onto a lightly floured work surface. Stamp out 20 circles using the larger pastry cutter. Use the smaller pastry cutter to cut a shallow circle inside each round, making sure you don't cut all the way through.
3. Transfer the circles to the baking sheet. Spoon tapenade mixture in the center of each round and place a mushroom on top.
4. Lightly brush the edges of the pastry circles with the yolk mixture. Bake for about 15 minutes until golden and puffed up. Serve hot or warm.
Chicken and Pesto Vol-au-vents
Omit the mushrooms. Substitute 3 tbsp pesto for the tapenade. Top with equal amounts of ¾ cup finely diced cooked chicken, then sprinkle with 2 tbsp pine nuts.
Smoked Salmon and Ricotta Vol-au-vents
Omit the tapenade mixture and mushrooms. Substitute 5oz (150g) hot-smoked salmon, flaked, with 4 tbsp ricotta cheese and 2 tbsp dill.