Place the carrots, onions, and garlic in a large bowl. Drizzle with the olive oil, then sprinkle with 1 teaspoon salt, cumin, and pepper. Toss to evenly coat, then spread in a single layer on the prepared baking sheets, ensuring that the vegetables do not crowd one another. Roast for 25 to 30 minutes, turning twice throughout, until the vegetables are tender and browned. Let cool on the pans for 10 minutes.