In a medium sized frying pan, cook the bacon until it reaches desired doneness. Remove and drain.
Add onion and mushrooms to the bacon drippings, and cook, stirring frequently, until onions are soft, about 5 minutes.
While onions are cooking, chop the bacon and set aside, and slice apple.
Once onion is done cooking, add the bacon back to the pan, add the maple syrup and apple cider vinegar. Boil it for 1 minute, and remove from heat to cool slightly.
Plate the spinach and apple individually and spoon warm dressing over plates, or serve in a salad bowl, tossed with the dressing. Serve immediately.