Preheat oven to 500°F. Peel 1 lb. parsnips and 1 lb. carrots; cut into thick sticks. On a large, rimmed baking sheet lined with foil, combine parsnips and carrots with 1 (10 oz.) package frozen pearl onions (thawed), 8 oz. halved brussels sprouts and 6 smashed and peeled garlic cloves. Toss vegetables with leaves of 1 sprig fresh rosemary (or 1 tsp. dried), 3 Tbsp. olive oil, 1/2 tsp. kosher salt and 1/4 tsp. black pepper until coated. Spread vegetables evenly on a baking sheet. Roast until tender and browned on edges, about 25 minutes.