Preheat the oven to 425 degrees F (220 degrees C). Place a rimmed baking sheet inside to preheat for 5 minutes.
Toss cauliflower with oil, cumin, salt, and pepper in a large bowl. Spread cauliflower carefully on the hot baking sheet. Set the bowl aside.
Roast in the preheated oven until tender and starting to brown, about 15 minutes. Stir and continue roasting until evenly browned, about 5 minutes longer.
Meanwhile, stir tahini and Sriracha sauce together in the bowl. Thin with a little water. Add hot cauliflower and stir gently to coat.
Plate cauliflower and garnish with cilantro and sesame seeds.