Preheat the oven to 190C/gas 5. Put the carrots, fennel and onion in a roasting dish and toss with 2 tablespoons of olive oil. Roast for 20 minutes, then add the garlic cloves. Stir everything thoroughly and return to the oven for 20 minutes more, until the vegetables are soft and browned. Remove the papery skins from the garlic cloves.