Heat the oven to 190°C (Gas Mark 5 / 375°F). Place the prepared vegetables in a large roasting tray, and add the olive oil, balsamic vinegar, maple syrup, and plenty of salt and black pepper. Mix well with a spoon until evenly coated. Place in the oven and roast for around 1 and a half hours, or until soft and slightly crispy on the edges.
An hour or so after putting the vegetables in the oven, put the pearl barley in a pan of water to boil - it will probably take around 30 minutes.
When the pearl barley is cooked, drain it, and add the walnuts. Serve with the fresh rocket and roasted vegetables, drizzled with any excess dressing.