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.