Preheat oven to 375°F.
Place quartered onions into a baking dish and toss together with oil, salt and pepper.
Roast for 15 minutes uncovered. Remove from oven, gently toss around and cover to foil. Place back into the oven and continue to roast for 30 to 40 minutes or until onions are fork tender.
While the onions roast, place butter into a nonstick pan and melt over medium-low heat. Add remaining onion and sprinkle sugar over. Season with salt and pepper. Caramelize onions for 20 minutes, then add garlic and stir. Continue to caramelize for 15 to 20 minutes or until the mixture has become dark, rich and sweet.
Once onions have roasted and caramelized, place both into a food processor along with the remaining ingredients and puree until smooth. Adjust seasoning.
Allow mixture to cool completely before placing in the refrigerator for at least an hour before serving with a side of crudité or crackers.