This does appear to be a mountain of mushrooms when first sliced, but they reduce by ¾ when cooked.
10 grams dried porcini mushrooms
2 cups boiling water
1 tablespoon olive oil
large knob of butter
2 large onions, thinly sliced
500 grams button mushrooms, thickly sliced
300 grams Portobello mushrooms, thickly sliced
2 teaspoons brown sugar
2 cloves garlic, crushed
1 tablespoon chopped thyme
1 bay leaf
2 tablespoons plain flour
1 cup white or red wine
3 cups good beef stock
¼ cup chopped flat-leaf parsley
sea salt and freshly ground pepper
8 slices baguette, toasted
2 cups grated Gruyère cheese
1 teaspoon dry mustard powder
1 teaspoon Worcestershire sauce
Soak the porcini mushrooms in the boiling water for 15 minutes then drain, reserving the soaking liquid, and roughly chop.
Heat the olive oil and butter in a large, wide saucepan. Add the onions, all the raw mushrooms and the porcini, sugar, garlic, thyme and bay leaf. Season and cook over a low heat for 25 minutes, stirring occasionally, until the onions are tender and the mushrooms are lightly golden. Do not let them catch and burn.
Sprinkle over the flour and mix well. Slowly add the wine, scraping the bottom of the pan to release any sticky bits and cook for 2 minutes. Stir in the reserved mushroom soaking liquid and the beef stock. Season and bring to the boil then simmer for 25 minutes. Remove the bay leaf and stir in the parsley.
Preheat the grill to its highest setting.
Combine the cheese, mustard powder and Worcestershire sauce in a bowl and season with freshly ground pepper.
Place the toasted bread on a baking tray and top with the cheese mixture. Place under the grill for 2-3 minutes or until bubbling and golden brown.
To serve: Ladle the soup into warm bowls and place one toast in each soup. Serve the remaining toasts separately.