Serve this dip with pita chips or carrot sticks. Making the recipe a day ahead allows the flavors to meld.
Place onion in a large bowl; drizzle with oil. Sprinkle with 1/2 teaspoon salt; toss to coat. Remove white papery skin from garlic head (do not peel or separate the cloves). Wrap in foil. Place onion and foil-wrapped garlic on a baking sheet. Bake at 425° for 1 hour; cool 10 minutes. Chop onion. Separate garlic cloves; squeeze to extract garlic pulp. Discard skins.
Combine onion, garlic, 1/2 teaspoon salt, sour cream, and remaining ingredients in a large bowl. Cover and chill 1 hour.
This is just an o.k. onion dip. It's good if you really are craving something like this, but don't want the fat. If I wanted to make an onion dip for a party, I would opt to make one of the hot vidalia onion dips from Southern Living or Coastal Living that are just delicious -- people just inhale them!