Preheat the oven to 200°C (180°C fan, gas 6).
Let the puff pastry thaw then cut out 16 x 6cm circles.
Cut out smaller circles from half of the circles with a 5cm cutter.
Brush the full circles with egg white and cover each one with a pastry ring. Place on a baking tray lined with baking paper and bake in the preheated oven for 15 minutes. Remove and allow to cool.
Meanwhile, peel and finely dice the onion and garlic.
Cook in melted butter until softened. Clean the mushrooms and finely chop then add to the pan. Stir until the mushroom liquid has evaporated.
Stir in the crème fraîche, salt and pepper and simmer for 3-4 minutes.
Wash the parsley, shake dry and chop finely.
Add the parsley, stir and season with salt and pepper.
Transfer the mushroom mixture to a bowl and allow to cool.
To serve, spoon the creamy mushrooms into the puff pastry shells and garnish with thyme.