Preheat your oven to 200°C/Fan 180°C/Gas 6. Halve the butternut squash and scoop out the seeds. Cut each half into 6-8 wedges and lay them on a baking tray (no need to peel unless you want to). Halve the tomatoes and pop them on the tray. Drizzle over 2 tbsp olive oil and dust with salt and pepper. Slide into the oven to roast for 30 mins.
While the veg roasts, fill and boil your kettle. Tip the pearl barley into a small pan. Add a pinch of salt, and enough boiling water to cover the barley by a few cms. Pop on a lid, set on a medium heat and simmer for 15-20 mins till the barley is tender but still has a little bite. Drain and shake dry.
Meanwhile, finely grate the lemon zest and set it to one side. Scoop the pesto into a small bowl. Add 1 tbsp olive oil and squeeze in the juice from half the lemon. Whisk together to make a dressing.
When the veg has cooked for 30 mins, remove the tray from the oven. Scatter over the pine nuts and slide back into the oven for 5 mins. The pine nuts will brown and the veg will be tender.
When the squash is ready, remove it from the oven and leave to cool slightly. Arrange the watercress on a platter or 2 serving plates. Scatter over the pearl barley and drizzle over half the pesto dressing. Top with the roast squash wedges and tomatoes. Drizzle over the remaining pesto dressing and sprinkle with the lemon zest. Serve with the zested lemon, cut into wedges, on the side for squeezing.