Heat the oil in a large, heavy-based saucepan over a medium heat. Stir in the onion, carrots, celery and garlic, then cook gently for about 10 minutes, stirring occasionally, until softened but not browned.
Stir in the barley and cook for 1 minute longer. Pour in the stock and bring to the boil. Add the thyme and lemon zest. Reduce the heat to very low, cover and simmer, stirring occasionally, for 20 minutes or until the barley is almost soft.
Add the peas, re-cover and simmer for a further 4–5 minutes until the barley is soft but not mushy and all the stock is absorbed. Stir in the lemon juice and season to taste.
Add the chicken pieces and rocket leaves to the pan and lightly toss together. Serve at once.