Preheat oven to 425°. Remove papery outer skin from garlic bulb, but do not peel or separate cloves. Cut top off of garlic bulb, exposing individual cloves. Brush cut cloves with 1 teaspoon oil. Wrap in foil. Bake until cloves are soft, 30-35 minutes.
Meanwhile, in a large skillet over low heat, heat remaining oil. Add the chopped onion; cook until golden brown, 15-20 minutes, stirring occasionally. Transfer to a food processor. Process until blended; set aside.
Place potatoes in a large saucepan; add water to cover. Bring to a boil. Reduce heat; cook, uncovered, until tender, 15-20 minutes. Drain; return to pan. Squeeze softened garlic onto potatoes; add butter, sour cream, cheese, milk, salt, pepper and onion. Beat until mashed. Freeze option: Place cooled mashed potato mixture in a freezer container and freeze. To use, partially thaw in refrigerator overnight. Microwave, covered, on high, stirring twice and adding a little milk if necessary, until heated through.