Preheat oven to 400 degrees (F). Remove excess papery skins from garlic bulbs and carefully slice off the top of the bulbs; place in a small oven-safe baking dish, drizzle with 3 teaspoons of olive oil. and wrap the bowl in heavy-duty foil. Bake for 40 minutes, or until the inner garlic cloves have softened. Set aside to cool.
Meanwhile, in a large skillet over medium-low heat, saute the onions in remaining 3 tablespoons of olive oil, stirring occasionally, for 25-30 minutes, or until golden brown. Transfer to a food processor or strong blender, cover, and process until blended; set aside.
Place potatoes in a large stockpot and cover with water; bring to a boil over high heat. Once at a rapid boil, reduce heat to medium-low, cover, and cook for 15 minutes, or until tender enough to pierce with a fork. Drain the potatoes well. Place potatoes in a large bowl; add in the roasted garlic cloves, butter, sour cream, cheese, heavy cream, salt, pepper and onion mix. Using a handheld electric mixer on low-speed, beat potatoes until light and fluffy. Taste and adjust salt and pepper as needed. Serve warm.
Notes
If you do not own a food processor, transfer the onions to a large cutting board and do your best to chop the caramelized onions into an "onion paste".
The garlic may be roasted ahead of time.
The onions may be caramelized ahead of time
To freeze: place completely cooled mashed potatoes in a freezer-safe container and freeze. When ready to eat, thaw in refrigerator overnight, then microwave, covered, on high in a microwave-safe bowl until heated through. Add a little cream if needed.
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