Preheat the oven to 200ºC / 400ºF / Gas 6. Place the aubergines and courgettes in a roasting tin
(pan). Add the onion and garlic, and drizzle over the olive oil. Roast for 35 minutes, turning
once, until tender and slightly charred.
Place half the roasted vegetables in a food processor or blender. Add the stock and process
until almost smooth. Pour into a large pan and add the remaining vegetables. Bring to the boil,
season and stir in the chopped oregano.
For the tzatziki, peel, seed and dice the cucumber. Place the flesh in a colander and sprinkle
with salt. Leave for 30 minutes. Mix the garlic with the vinegar and stir into the yogurt Pat the
cucumber dry on kitchen paper and fold it into the yogurt. Season to taste and stir in the mint.
Chill until required. Ladle the soup into bowls and garnish with mint sprigs. Hand round the bowl
of tzatziki so that your guests can add a dollop or two to their soup.