Adjust oven rack to lowest position and heat oven to 475 degrees. Toss cauliflower with 2 tablespoons oil, ½ teaspoon salt, ½ teaspoon pepper, and ¼ teaspoon cumin. Arrange cauliflower in single layer in rimmed baking sheet and roast until just tender, 10 to 15 minutes; set aside.
Heat remaining 2 tablespoons oil in large saucepan over medium heat until shimmering. Add onion and ¼ teaspoon salt and cook until softened and lightly browned, 5 to 7 minutes. Add rice, garlic, cinnamon, and remaining ¼ teaspoon cumin and cook, stirring frequently, until grain edges begin to turn translucent, about 3 minutes.
Stir in water and bring to simmer. Reduce heat to low, cover, and simmer gently until rice is tender and water is absorbed, 16 to 18 minutes.
Off heat, lay clean dish towel underneath lid and let pilaf sit for 10 minutes. Add roasted cauliflower to pilaf and fluff gently with fork to combine. Season with salt and pepper to taste. Transfer to serving platter and sprinkle with pomegranate seeds, cilantro, and mint. Serve.