Preheat oven to 450°. while oven is preheating add cauliflower florets to a bowl. In another small bowl, combine oil, salt, coriander, oregano, and pepper and toss it with cauliflower to evenly coat cauliflower.
Arrange cauliflower in an even layer on a baking tray. Roast cauliflower for 20 minutes, turning sheet and lightly tossing cauliflower halfway through.
Meanwhile, in a small bowl, whisk together tahini, vinegar, and water until smooth.
When done, remove the cauliflower from the oven and add to a large bowl (there will be lots of bowls to wash up but totally worth it) In a large bowl, toss cauliflower with tahini sauce, sesame seeds and chili flakes (chili flakes are optional) to coat evenly. Serve.