The group of old ladies that were brought to teach me how to thread tomatoes together into a fine bunch were the funniest lot of pensioners I have ever met.
First make the croutons. Preheat the oven to 180C. Place the bread in a medium bowl along with the oil, vinegar and 1 teaspoon of salt. Place a griddle pan on a high heat, add the croutons and cook for 2 minutes, turning until all sides are slightly charred and starting to crispen. Transfer from the pan onto a baking tray and place in the oven for about 12 minutes, until golden brown and crispy. Set aside to cool.
Place the tomatoes, garlic, celery, onion, watermelon, bread, passata and 10 grams of basil in a blender or large bowl along with 1½ teaspoon of salt and a good grind of black pepper. Blend until smooth and then, with the blender still going, add the vinegar and olive oil. Refrigerate until needed.