Ingredients (Serves 4)
Preheat the oven to 200°C, 400°F, Gas 6. Season the Portobello mushrooms and fry them with a clove of garlic and the thyme in 2 tbsp of oil for about 45 seconds on each side. Transfer to the oven and bake for 5 minutes. Remove, cover with clingfilm gill-side down and leave to cool.
Reduce the oven temperature to 190°C, 375°F, Gas 5. Sauté the shallots and remaining garlic in the rest of the oil for 2 minutes. Add the mixed mushrooms and sauté for 2 minutes more. Drain the mushrooms (reserve the juices), mix with the herbs, season, then leave to cool.
Bring the stock and the reserved liquid from the mushrooms to the boil slowly, together with the butter and flour, stirring with a balloon whisk until the mixture thickens. Away from the heat, add half the cheese to the sauce and when it has melted, stir in the sautéed mushrooms and season to taste.
Brush three sheets of filo pastry with a little melted butter and place one on top of each other at right angles to the sheet beneath. Make three more stacks in the same way. Place a Portobello in the centre of each stack, top with the mushroom mixture and the remaining cheese.
Gather the corners of the pastry around the mushroom to enclose the topping and seal with melted butter. Bake for 15 minutes until the pastry is golden and crisp. Serve immediately.