Cut the sweet potato into 1.5cm chunks and place on a baking sheet. Drizzle with a little olive oil and toss to coat the pieces, then season with the cumin and some salt and pepper. Place the lemon on the baking sheet with the sweet potato and roast in the oven for 30 minutes.
Hold the pomegranate cut side down over a bowl and bash the top with a wooden spoon to release the seeds. Reserve the other half for another recipe. Squeeze the juice from the roasted lemon halves into a small bowl and whisk in the 4 tablespoons of olive oil.
Put the sweet potato into a large bowl and add the chickpeas and dressing. Gently fold in the crumbled feta cheese, pomegranate seeds and rocket and transfer to a serving bowl.