In a large skillet, cook bacon until crisp. Remove to paper towels. Drain, reserving 2 tablespoons drippings. , In same skillet, combine vinegar, brown sugar and reserved drippings. Bring to a boil; cook and stir until sugar is dissolved. Cool slightly., Meanwhile, in a serving bowl, combine spinach, apples, onion and bacon. Drizzle with warm dressing; toss to coat. Serve immediately.