Cook the bacon over medium high heat until crispy. Remove the bacon from the grease and allow it to drain on paper towels. Drain away all but about 2 tablespoons of the remaining bacon grease in the pan. Return the pan to medium heat and add the diced onion. Cook 6 to 8 minutes, or until tender. Turn off the heat and stir in the mustard, brown sugar, and vinegar. Add the bacon back to the pan and mix well. Serve warm over your favorite brats, hot dogs, or other sausage.