1. Warm your oil in a large pan (it needs to be big, this makes a lot of soup). Soften your onions, celery and garlic over a medium heat for about 10-15 minutes. Don't let the vegetables colour.
2. Add your carrots and cook gently for another 10 minutes until a little soft. Stir often so they don't stick.
3. Pour over hot, weak vegetable stock. Bring to the boil and then simmer for 25 minutes.
4. For the last 10 minutes, add the lettuce and peas.
5. When 5 minutes is left, add the spinach and mint.
6. Blend the soup. You won't need to season with salt if you used a shop bought stock, but if it's homemade you may need it.