Preheat oven to 425Toss 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 minutes.
Remove from oven, sprinkle Parmesan over, toss to combine, and roast until cauliflower is tender, 10-12 minutes longer.