Soak the dried mushrooms overnight then rinse several times to get all the sand out. Drain them out on paper towel.
Place the 6 pieces of puff pastry the pastry tray. Brush the egg wash on top and score lightly with a knife in a criss-cross design.
Bake the puff pastry squares at 180°C for 25 to 30 minutes until the dough has puffed and the top has browned.
When the puff pastry squares are baked let them cool down on the oven rack.
Peel and cut the carrots and celery root in 5cm x 7mm x7mm large matchstick pieces (batonnet).
Wash and cut the ends of the string beans. Cut the in half lengthwise to match the other batonnets of vegetables.
Place the carrots, celery root and string beans in the perforated gastro container and steam them for about 6 minutes. Once cooked, drop them in a bowl of cold water for 1 mn. Drain them out and set them aside.
Heat up the sauté pan and add the butter. Place the vegetables and the drained mushroom in the melted butter and reheat them for 4 minutes. Season them well.
Meanwhile open the puff pastry square in half
Spoon the vegetables on to the opened, warm puff pastry and serve hot.