Heat some butter in a big pot and sauté the onion and garlic.
Then add the mushrooms and thyme.
When they are almost cooked, add the flour. Fry briefly and then add the stock.
Bring to boil and stir in the cream.
Simmer for about half an hour and season to taste with salt and pepper.
Meanwhile, preheat the oven to 220 degrees Celsius / 430 Fahrenheit top and bottom heat.
Remove the thyme sprigs from the soup. Pour the soup into an ovenproof dish, brush the sides with the egg and milk mixture and lay the puff pastry on top.
Brush the pastry with the rest of the egg and milk mixture, sprinkle with a little sesame (or a mixture of sesame and poppy seeds), thyme and/or Parmesan and bake for 8-10 minutes until cooked and golden brown. Serve immediately and enjoy!