Put the tomatoes, garlic, peppers, vinegar and passata into a food processor or blender and whizz until smooth.
Pour in 500ml cold water and whizz again, adding more water to adjust the thickness to your taste. Season with freshly ground black pepper, transfer to a large jug and chill in the fridge for 30 minutes to 2 hours.
To serve, divide between bowls, scatter over the croutons and top with the cucumber, chilli and onion.