Arrange racks in the upper and lower thirds of your oven. Heat oven to 425oF.
Prepare the cauliflower: In a large bowl, whisk together olive oil, cumin, paprika, salt, coriander, turmeric, black pepper and cayenne. Add cauliflower and onion, and toss until well coated. Spread mixture in a single layer on a rimmed baking sheet.
Roast vegetables on the top oven rack until they are golden brown, slightly crisp and tender, 30 to 40 minutes, stirring once or twice. If the vegetables look dry as they roast, drizzle with a little more olive oil.
As they cook, prepare the cumin tahini sauce: In a small bowl, add all sauce ingredients and whisk or mix together until smooth.
Scatter parsley on top of the roasted vegetables and serve with cumin tahini sauce.