Preheat oven to 400°. Put potatoes in a pot of salted water. Bring to a boil, then simmer until just tender, 10 to 15 minutes. Drain and put in a large bowl. Meanwhile, blanch onions in boiling water 2 minutes; rinse to cool, then peel.
Add onions, butter, oil, 1 tsp. salt, and 1/2 tsp. pepper to potatoes and stir. Divide between 2 rimmed baking sheets and bake, stirring occasionally, until potatoes are tender and golden brown with crisp edges, about 25 minutes. Transfer to a shallow serving dish.
Beat crème fraîche and heavy cream in a bowl with an electric mixer until stiff peaks form. Stir in chives, bacon, and remaining 1/4 tsp. salt and 1/8 tsp. pepper.
Top potatoes with bacon cream and gently stir to coat.
Make ahead: Through step 1 or 2, 1 day, chilled and covered. Bring to room temperature before continuing recipe, and reheat baked potatoes at 375° for 30 minutes before topping with bacon cream. Bacon cream: Up to 2 hours ahead, chilled.
Note: Nutritional analysis is per serving.