1. Peel the sweet potato and roughly dice. Boil until soft but not mushy, cover the pan and set aside.
2. Boil the carrots for a just a few minutes making sure they remain crunchy. Add the carrots to the sweet potatoes.
3. Drain the water from the sweet potatoes and carrots and place together in a bowl.
4. Slice the tops off the tomatoes, then scoop out and discard the seeds. Roughly chop the flesh.
5. Line a glass bowl with the lettuce leaves. Mix together the sweet potatoes, carrots, chick-peas (garbanzos) and tomatoes and place in the bowl.
6. Blend together all the dressing ingredients and beat using a fork.
7. Garnish with the walnuts, sultanas (golden raisins) and onion rings. Pour the dressing over the salad or serve it in a separate bowl, if wished.