Preheat oven to 425 degrees F. In a bowl, toss cauliflower, onions, thyme and garlic with olive oil, salt and pepper. Season with salt and pepper
Spread veggies evenly on a parchment-lined baking sheet. Roast, tossing occasionally, until almost tender for about 15-20 minutes.
Remove from oven, sprinkle with Parmesan, toss to combine, and continue to roast until cauliflower is tender and a bit caramelized (about 10-12 minutes longer).