Preheat oven to 425°. Toss cauliflower florets on a large rimmed baking sheet with onion, thyme, garlic, and oil; season with salt and pepper. Roast, tossing occasionally, until almost tender, 35-40 minutes. Sprinkle with Parmesan, toss to combine, and roast until cauliflower is tender, 10–12 minutes longer.