Heat the oil in a large saucepan. Add the onion and fennel and cook over a high heat for 5 minutes, stirring frequently, until slightly softened. Stir in the garlic and mushrooms. Continue to cook, stirring frequently, for a further 5 minutes. Pour in the boiling water and stir in the stock cubes. Bring back to the boil, then reduce the heat and simmer the soup, uncovered, for 10 minutes.
Meanwhile, preheat the grill to moderate. To make the croutons, brush the slices of ciabatta bread lightly on both sides with the oil and toast them under the grill for about 1 minute on each side or until golden. Cut the bread into cubes and place in a bowl. Add the parsley and mint and toss well.
Taste the soup and add seasoning if necessary. Ladle the soup into bowls. Sprinkle with the parsley and mint croutons and serve at once.
Some more ideas…
For a rich and creamy version of this soup, thicken with an egg yolk and cream mixture. Whisk 1 egg yolk with 2 tbsp cream until lightly mixed. When the soup is cooked and croutons prepared, remove the pan from the heat and stir a ladleful of soup into the mixture. Pour the mixture into the pan and stir over a low heat for about 30 seconds. Do not allow the soup to get too hot and start to simmer or the egg yolk will curdle. Serve at once. * Use 3 finely chopped celery stalks instead of the fennel.
In Asian cultures, mushrooms are renowned for their ability to boost the immune system, and the Chinese have used mushrooms medicinally for over 6000 years. Mushrooms are a useful source of the B vitamins B6, folate and niacin, as well as copper. * Brushing slices of bread with oil and toasting under the grill before cutting them into cubes is a good way to make low-fat, crisp croutons.