Bring a large pot of water to a boil and add 1 tablespoon salt. Add the green beans and cook until just tender, 4 to 5 minutes. Drain and run under cold water to cool; transfer to a serving bowl.
Meanwhile, in a medium skillet, cook the bacon over medium heat until crisp, 6 to 8 minutes; transfer to a paper towel-lined plate. Let cool, then break into pieces.
Discard all but 2 tablespoons of the bacon drippings from the skillet and return to medium heat. Add the shallots and cook, stirring, for 1 minute. Stir in the vinegar, mustard, oil, ¾ teaspoon salt, and ¼ teaspoon pepper. Add to the green beans, along with the bacon, and toss to combine