Preheat oven to 425. 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 Tbsp. olive oil; season with kosher salt and freshly ground black pepper. Roast, tossing occasionally, until almost tender, 35-40 minutes. Sprinkle with 1/2 cup grated Parmesan , toss to combine, and roast until cauliflower is tender, 10-12 minutes longer.
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