Preheat oven to 200 C / Gas 6 and lightly grease a baking tray with a tablespoon of olive oil.
Rinse cauliflower and divide into equally sized florets. Place into a bowl and coat with the remaining olive oil and the za'atar and salt. Toss well. Place cauliflower in a single layer onto the prepared baking tray.
Roast for 35 to 45 minutes in the preheated oven. After 20 minutes turn once.
While the cauliflower is roasting, prepare the tahini sauce: mix together yoghurt, tahini and lemon juice until smooth and well incorporated. Begin stirring in warm water, about a tablespoon at a time, until a drizzling consistency is achieved. Taste and add more salt or lemon juice, if desired.
Also toast the sesame seeds: place a small frying pan over medium heat. Add sesame seeds and toast until golden, about 3 minutes. Remove seeds from pan and set aside.
Remove cauliflower from the oven when tender and golden brown. To serve, place cauliflower in an even layer in a shallow dish or bowl. Drizzle with the tahini dressing, then sprinkle with the sesame seeds and sumac.
Sumac has a lemony flavour that contrasts well with the creamy tahini sauce. If you don't have sumac, you can sprinkle additional za'atar over the finished dish instead.
You can serve the cauliflower hot or at room temperature.