Preheat oven to 400 F and line two large baking sheets with parchment paper. Place the cauliflower on one baking sheet and drizzle with 1.5 tbsp of the olive oil, salt, and pepper. Toss to coat. Roast for 30-40 minutes, or until browned and crispy around the edges.
Place the chickpeas on the second baking sheet. Drizzle with olive oil and sprinkle with the salt and za’atar. Roast until browned and crispy, about 25-30 minutes, keeping in mind that they will crisp up more once out of the oven.
Meanwhile, heat the remaining 1 tbsp of olive oil in a large pot over medium heat. Add the onions and garlic and cook, stirring occasionally, for five minutes. Add the celery, a pinch of salt and pepper, and spices, and cook a few minutes more. Add the broth and bring the soup to a low simmer.
Once the cauliflower is ready, add it to the pot and remove from heat. Stir in the tahini.
Blend the soup in two batches. Add half of the soup to the jar of the blender and place half of the cilantro on top (we are adding the cilantro now instead of earlier to allow it to retain its beautiful green colour!). Blend until smooth, pour it into a a large container or bowl, and repeat with the second half of the soup. Return the soup to the original pot and warm it up more, if desired. Right before serving, add the lemon juice.
Ladle into bowls and garnish with roasted chickpeas.