Boil potatoes in a pot of salted water until tender, 15-20 minutes.
Cook bacon in oil in a saucepan until crispy.
Remove cooked bacon to paper towels to drain, leaving oil in pan.
Add onion to pan; saute until softened.
Add flour; stir until paste-like.
Stir in vinegar and brown sugar.
When mixture is thick, return bacon to skillet.
Simmer mixture for 1-2 minutes.
Add potatoes to pan with bacon mixture; stir gently until potatoes are coated.
Transfer potato salad to a serving bowl; sprinkle with scallions.
Serve potato salad warm.