Cook bacon in a large skillet over medium heat until crisp (5 to 7 minutes); drain on paper towels, then crumble. Add onion and salt to the drippings in the pan and cook over medium heat, stirring often, until tender and browned (about 3 minutes). Add water and mustard, scraping up any browned bits, then add Brussels sprouts and cook, stirring often, until tender (4 to 6 minutes). Stir in vinegar and top with the crumbled bacon.