Classic Roasted Root Vegetables

3 pounds root vegetables, such as carrots, parsnips, sweet potatoes, turnips, rutabaga and beets1 small red onion1/4 cup olive oil1 1/2 teaspoons kosher salt1/4 teaspoon freshly ground black pepper2 tablespoons coarsely chopped fresh rosemary (or thyme, oregano, parsley) leavesArrange a rack in the middle of the oven and heat the oven to 425°F.Peel (or just scrub) 3 pounds root vegetables, then cut them into rough 1-inch chunks. Cut 1 small red onion into 1-inch chunks. Place the root vegetables and red onion on a rimmed baking sheet. Drizzle with 1/4 cup olive oil, sprinkle with 1 1/2 teaspoons kosher salt and 1/4 teaspoon ground black pepper, and toss to evenly coat. Spread out in an even layer.Roast for 30 minutes. Meanwhile, chop until you have 2 tablespoons coarsely chopped rosemary leaves.Remove the baking sheet from the oven, sprinkle the vegetables with the rosemary, and toss to combine. Spread back out in an even layer. Continue to roast until the vegetables are tender and caramelized, 10 to 15 minutes more.