In a large bowl, add onion quarters and toss with olive oil and 1/2 teaspoon of salt. Spread onions on a large baking sheet.
Remove the papery skin from the garlic but do not peal or separate the close. Slice off the very tip of the garlic head (so that the cloves are exposed.) Rub a little bit of oil, from the bowl the onions were tossed in on the exposed tops of the garlic cloves. Wrap the garlic head in foil and place on baking sheet with the onions. Bake at 425 for 1 hour.
Remove from the oven and allow onions to cool enough so you can handle them - at least 10 minutes. Chop the onion and place in a medium bowl.
Remove the foil from the garlic head and squeeze the soft garlic into the bowl with the chopped onions. Discard the skins.
Add sour cream, parsley, lemon juice and additional salt and pepper with the onions and garlic. Stir to mix. Cover and chill for 1 hour.