Preheat oven to 425°F (220°C) and line a baking sheet with parchment paper or a silicone baking mat.
Cut the cauliflower into bite-sized florets of similar size for even roasting.
In a large bowl, toss cauliflower florets with olive oil, cumin, smoked paprika, turmeric, salt, and pepper until evenly coated.
Spread cauliflower in a single layer on the prepared baking sheet, avoiding overcrowding.
Roast for 25 minutes, flipping the florets halfway through for even browning.
While cauliflower is roasting, make the tahini sauce: whisk together tahini, lemon juice, garlic, and salt. Gradually add water until the sauce reaches a creamy, pourable consistency.
Once roasted, allow cauliflower to cool slightly, then transfer to a serving dish.
Drizzle with tahini sauce or serve the sauce on the side.
Garnish with chopped parsley if desired, and serve warm.