Preheat an oven to 300 degrees F (150 degrees C).

Cut off the top of the head of garlic to expose the cloves, trimming about 1/4 inch off of the top of each clove. You may need to trim individual cloves along the sides of the head. Brush the cut cloves with a small amount of olive oil, then nestle the head into a piece of aluminum foil.

Bake in the preheated oven until the cloves are tender and nicely browned, about 1 hour. Remove, and allow to cool to room temperature.

Once cool, squeeze the garlic cloves out of their skins and into a mixing bowl. Mash well with a wire whisk, then add the sour cream, mayonnaise, green onions, vinegar, salt, and pepper. Whisk until evenly blended, then refrigerate 2 to 4 hours to allow the flavors to blend.