Heat oven to 350 F.
Combine onions, garlic and olive oil. Place in ovenproof baking dish; cover tightly with foil and bake 45 minutes until tender. Remove from oven and set aside.
In large saucepan, cover potatoes with water and bring to boil over high heat. Reduce to medium heat and cook 20-30 minutes until tender. Drain well. Do not let cool.
In bowl, mash or whip potatoes, roasted onions, butter, sour cream, salt and pepper until fluffy and combined.