Preheat the oven to 200°C (400°F) and line a baking tray with parchment paper.

Roll out the puff pastry on a floured surface. Use a round cutter to cut out circles for the bases of the vol-au-vents.

Use a smaller cutter to cut out the centers of half of the circles to create the pastry rings.

Place the pastry bases on the prepared tray and place the rings on top. Brush with beaten egg to give a golden finish.

Bake for 15-18 minutes until puffed and golden.

While the pastry bakes, heat butter and olive oil in a pan. Add the onions and cook for 3-4 minutes until softened.

Add the sliced mushrooms and cook until tender and golden brown.

Pour in the vegetable broth and cook until most of the liquid has evaporated. Stir in the heavy cream, thyme (if using), salt, and pepper. Cook for 2-3 minutes until the sauce is creamy and thickened.

Once the pastry shells are baked and golden, carefully spoon the mushroom mixture into each shell.

Serve immediately as an elegant appetizer or light snack.