Wash hands with soap and water.
Scrub the onion with a clean vegetable brush under running water. Thinly dice the onion.
Gently rub Brussels sprouts under cold running water. Slice the Brussels sprouts in half and then thinly slice each half.
In a large deep skillet, cook bacon over medium heat until browned and crisp.
Wash hands with soap and water after handling uncooked bacon.
After bacon is browned and crisp, remove from the pan and set aside.
Once bacon has cooled, crumble into small pieces.
Add the onion to the hot bacon drippings and season with salt. Cook over medium heat until tender, about 5 minutes.
Add the water and Dijon mustard to the onions. Increase the heat to medium high and stir until the water comes to a boil.
Add the Brussels sprouts and vinegar to the skillet. Cook, stirring often, until the Brussels sprouts are tender, about 4 to 6minutes.
Remove skillet from heat and toss Brussels sprout mixture with bacon and blue cheese crumbles.