In a skillet set over moderate heat cook the bacon until it is crisp. Transfer the bacon to a plate lined with paper towel and let it drain. Pour off all but 2 tablespoons fat from the skillet.
Add the onion to the skillet and cook it, stirring occasionally, for 3 minutes, or until it is wilted. Add the vinegar, mustard, sugar and salt and pepper, to taste and bring the mixture to a boil, stirring. In a serving bowl combine the spinach and apple. Add the hot dressing and toss to combine and serve.