Preheat oven to 425.
Remove papery skin from garlic head, but do not peel or separate cloves. Place on a piece of foil and drizzle with olive oil. Wrap in foil and place on a baking sheet.
Place each onion on a piece of foil, cut side down. Drizzle with olive oil, but do not wrap with foil. Place on baking sheet with garlic and bake for 30 minutes. Remove from oven and wrap onions with foil. Return to oven for another 30 minutes.
After onions and garlic have cooled awhile, separate garlic cloves and squeeze to extract garlic pulp. Discard the skins. Put all ingredients in food processor. Process until it is well mixed and a smooth consistency.
You can also use the garlic cloves that are already peeled. We buy a large bag from Costco and freeze them. Then we just take out whatever amount we need and use it. For this recipe, just put a handful on the piece of foil and continue as described. No need to extract garlic pulp from the skins this way!