Cut the sweet potatoes into 3cm-thick slices and boil in water for 5-8 minutes until just cooked, but still with a bit of bite. Add the beans for the last 2-3 minutes. Drain and cool, then peel the skins off the sweet potato and cut into chunks.
Meanwhile, heat the oil in a large saucepan and fry the onion, garlic, chilli and rosemary for 5 minutes until soft.
Add the barley and 600ml (1pt) boiling water. Simmer uncovered for 25-30 minutes until tender. Allow to cool.
Add the beans and sweet potato to the barley.
Whisk the dressing ingredients together and pour over.
Chill until ready to serve.
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