Preheat the oven to 170°C (325°F/Gas 3). Place the garlic, cut-side down, in a roasting tray and drizzle over the oil. Roast for 15–20 minutes or until tender and golden — you may need to turn the garlic if it colours too quickly. Allow to cool, then peel. Place the cloves in an upright blender with the oil from the roasting tray and blend until smooth.
This can be made 3 days in advance and stored in an airtight container in the fridge.