Heat the oven to 200°C/400°F. Toss the cauliflower, red onion, za'atar and olive oil together on a baking sheet. Place in the oven and roast for 20-30 minutes.
Spread the hummus on a serving platter or on individual bowls. Add the watercress and olives to the tray with the cauliflower and toss together then place on top of the hummus. Serve sprinkled with pomegranate seeds.
Nutrition information is an estimate and will vary depending on the exact ingredients used.