In a Dutch oven or large stockpot, add potatoes and 2 tablespoons salt. Add cold water to cover potatoes by 1/2 inch. Over medium-high heat, bring to boil and partially cover pot. Cook until potatoes are tender and a fork can be easily slipped into the center, stirring once or twice, about 10 to 15 minutes. Drain well, tossing in a colander to remove excess water.
In a medium-sized skillet medium heat, cook bacon until crispy, about 7-10 minutes. With a slotted spoon, remove bacon and set aside on a paper towel-lined plate to drain.
Make the roux by adding vegetable oil to bacon fat and heat until a pinch of flour sprinkled into the oil just begins to bubble. Whisk flour into remaining bacon fat, and continue to cook and whisk until a thick paste forms, about 1 minute. Remove from heat and allow to cool.
In a medium-sized saucepan combine chicken broth, vinegar, onions, 1 teaspoon salt, 1/2 teaspoon pepper, sugar, and mustard. Bring to a boil, and cook uncovered until onions are tender, about 10 minutes. Add roux to broth mixture, 1 tablespoon at a time, until the sauce thickens. Discard any remaining roux.
In a large bowl, add warm potatoes and slowly add broth mixture, a little at a time, allowing time for the potatoes to absorb the mixture. Add bacon and toss to combine. Garnish with scallions. Serve potato salad warm.