Bring a large pot of water to a boil and season with sea salt. Add the green beans and cook until just crisp-tender, 2-3 minutes.
Drain and plunge into a bowl of ice water to cool and preserve the bright green color of the beans; transfer to a bowl.
Meanwhile, in a large skillet, cook the bacon over medium heat until crisp, 3-4 minutes; transfer to a paper towel-lined plate. Let cool, then break into pieces.
Discard all but 1 tbsp 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, sea salt, and freshly cracked pepper, to taste.
Add the green beans back to the pan, toss to coat evenly, and re-heat the green beans. Pour into a serving bowl. Enjoy.