Heat oil in a large nonstick skillet over medium heat; add onions and cook, stirring often until caramelized.
Meanwhile, boil potatoes in a large saucepan for 18 to 22 minutes or until tender.
Drain potatoes; place in a large bowl and mash. Add the sour cream, butter, milk, thyme, salt and pepper (to taste) and mash until blended. Stir in caramelized onions.
Transfer to a serving bowl. Sprinkle with cheese and bacon.