Cook bacon in a large skillet over medium-high heat until crispy. Remove bacon from skillet and allow to drain on paper towels. Pour off all but 2 tablespoons of the bacon grease in the skillet. Add the sliced onions to the skillet and cook over medium heat. Cook onions 6 to 8 minutes, or until tender and starting to caramelize. Remove skillet from heat and stir in the mustard, brown sugar, and vinegar. Add the bacon back to the onion mixture in the skillet and mix well. Serve warm.