1. Preheat oven to 425°.
2. 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 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.
3. Combine onion, garlic, remaining 1/2 teaspoon salt, sour cream, parsley, and lemon juice in a large bowl. Cover and chill 1 hour.
Freshly squeezed lemon juice is ideal. Those lemon-shaped plastic bottles do contain real juice, but after it's reconstituted and mixed with preservatives, the taste is notably off, not fresh, a bit harsh and thin. It lasts for months but doesn't really add that divine fresh-lemon essence.