Cut 1 head cauliflower into florets; toss on a large rimmed baking sheet with 1 sliced medium onion, 4 thyme sprigs, 4 unpeeled garlic cloves, and 3 tablespoons olive oil;
Season with kosher salt and freshly ground black pepper. Roast, tossing occasionally, until almost tender, 35-40 minutes.
Sprinkle with ½ cup grated Parmesan, toss to combine, and roast until cauliflower is tender, 10-12 minutes longer.
