Heat oil in pan. Chop onion, celery and carrot and fry for five minutes until soft Add tomatoes and stock. Season with pepper. Bring to boil and simmer 15 minutes. Pass raspberries though sieve to make purée.
Add raspberry purée lemon juice and sugar to the soup and check the seasoning. Add salt and pepper if needed. Blend until smooth. Serve with swirl of natural yoghurt and a sprig of mint