Preheat the oven grill.
Halve the tomatoes and place them cut side up on a baking sheet.
Roast until the edges begin to brown.
Remove from the oven.
Preheat the oven to 190 °C.
Heat the olive oil, garlic and herbs in a large pan.
When hot add the onions, celery, fennel and chilli and stir-fry until the vegetables just begin to soften.
Transfer the vegetable mixture to an oven pan and roast for 30 minutes uncovered.
Remove from the oven and process the mixture along with the tomatoes, tomato paste, sugar and lemon juice in a food processor until a puré forms.
Press the mixture through a sieve and pour into a saucepan.
Season with salt and add more water if the soup is too thick.
Heat the soup once more just before serving and spoon a dollop of basil purée in each soup bowl.
In a mortar, pound 60 g fresh basil with 2 ml salt using a pestle.
Add 60 ml olive oil and 10 ml balsamic vinegar and stir well.
Alternatively, purée all the ingredients in a food processor or blender to form a paste.