I cut the apples in half and use a mellon baller to remove the core, leave the skin on for color. Then slice very thin.
In an 8 to 10 inch skillet, cook diced bacon till crisp, drain on paper towels. Drain skillet reserving 2 tablespoons drippings. To skillet return dripping, Add Vinegar and Brown sugar. Bring to a boil and cook till sugar disolves. Cool slightly. Since I am a dressing hog I usually double this part.
In a large bowl combine Spinach, Apples, Onion and Bacon pcs. Drizzle with warm dressing. SERVE IMMEDIATELY.