Pre-heat the oven to 180 C degrees.
Place the sweet potato and onion in a large baking tray, add the oil and spices and mix coating the vegetables well. Place in the oven and bake for 30 minutes, shaking the tray once or twice, be careful not to burn the onions as they tend to cook faster. Remove from the oven let stand for 5 minutes. Spread the vegetables on a serving plate, add the beans if using, then drizzle the tahini sauce and sprinkle with zaatar.
Place all ingredients in a bowl and whisk until a thick but runny consistency, adding more water if needed.