Tip the barley into a small pan and sprinkle in a pinch of salt. Pour in 150ml boiling water. Return the pan to the boil and cover with a lid. Reduce to a simmer and cook for 25 mins, till the water has been absorbed and the barley is tender with a slight bite. Meanwhile, heat your grill to high.
Halve the peppers, and scoop out the seeds and membrane. Thinly slice the flesh. Peel the red onions and cut into thin wedges. Finely chop the rosemary leaves. Scatter both the peppers and onions onto a baking tray, and scatter over the rosemary. Toss everything together with 1 tbsp oil and a pinch of salt and pepper. Slide under the grill for 8–10 mins, turning halfway, till the veg is softened and caramelised at the edges.
Meanwhile, peel the garlic and crush or finely chop it. Slide it into a small bowl. Grate in the lemon zest and squeeze in the juice. Sprinkle in a pinch of sumach and a little salt and pepper. Whisk in 1 tbsp olive oil to form a dressing.
Dice the tomatoes and slide them into a large bowl. Drain the pearl barley if needed and tip into the bowl with the chopped tomatoes. Add the grilled veg and spoon over the lemon and sumach dressing. Fold everything together till well combined.
Divide the salad between a couple of plates. Crumble the feta over the top and finish with extra sprinkles of sumach.