1. Preheat the oven to 425°. In a large baking dish, toss the onion with the carrots, fennel and 2 tablespoons of the olive oil. Season with salt and pepper. Roast the vegetables for 20 minutes, or until crisp-tender and golden.
2. In a small bowl, stir the pesto with the remaining 1 tablespoon of olive oil. Add the pesto to the vegetables; toss to coat. If using the olives, scatter them on top. Bake the vegetables for 10 minutes longer, or until tender and browned. Transfer to a bowl and serve hot or at room temperature.
Notes: Variation Serve tossed with pasta or as a side dish for seared salmon, steak or pork.