Preheat oven to 450°. Mix parsnips, chopped rosemary, garlic, and oil on a large rimmed baking sheet. Season with salt and pepper and toss to coat. Spread out in a single layer. Scatter rosemary sprigs over.
Roast for 10 minutes; turn parsnips and roast until parsnips are tender and browned in spots, 10–15 minutes longer. Crumble leaves from rosemary sprigs over; discard stems and toss to coat. Sprinkle 1/2 tsp. cumin over. Season to taste with salt, pepper, and more cumin, if desired.
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