Preheat oven to 425 degrees. In a medium bowl, toss onions with 1 tablespoon olive oil; season with salt and pepper. Spread onions on a rimmed baking sheet, and roast until translucent, 15 to 20 minutes.
In same bowl, toss brussels sprouts with remaining 2 tablespoons olive oil; season with salt and pepper. Add to baking sheet with onions, and toss to combine. Roast, tossing occasionally, until vegetables are tender and caramelized, 25 to 30 minutes.