Preheat the oven to 400°F. Place in a baking dish:
Drizzle over the garlic and onion:
Cover the dish tightly with foil and roast until the garlic and onion are very soft, 1 hour to 1 hour 15 minutes. Cool slightly. Squeeze the garlic pulp from the skins into a food processor. Peel the onion and discard the skin and root end. Add to the food processor, along with any oil left in the baking dish. Pulse to combine. Add:
Process briefly to blend. If desired, stir in:
Taste and season with:
Serve with: