Preheat oven to 425°.
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.