Heat the olive oil in a large pot over medium-high heat. Add the onion and garlic and cook until soft, about 3 minutes. Add 1 to 2 teaspoons herbes de Provence and 1 teaspoon salt. Add the eggplant, zucchini and bell pepper and cook, stirring, 5 minutes. Add the tomatoes and their juices, the broth and half of the basil. Bring to a boil, then reduce the heat and simmer 20 minutes.
Preheat the broiler. Brush the bread with olive oil; sprinkle with the remaining 2 teaspoons herbes de Provence and salt to taste. Broil until lightly toasted. Top with the cheese, then broil until melted.
Puree about half of the soup in a blender, then return to the pot. Season with salt and pepper. Ladle the soup into bowls and top with the cheese toasts and the remaining basil.
Photograph by Antonis Achilleos
When blending hot liquid, first let it cool for five minutes or so, then transfer it to a blender, filling only halfway. Put the lid on, leaving one corner open. Cover the lid with a kitchen towel to catch splatters, and pulse until smooth.