1. Combine bacon, mayonnaise, onion and vinegar. Refrigerate until ready to use.
2. Combine roast beef, tomatoes, chili powder and brown sugar. Keep warm until ready to use.
3. Spread one side of each slice of bread with mayonnaise mixture. Top 4 slices evenly with spinach and beef mixture. Cover each with second slice of bread. Cut in half and serve.
Nutritional information per serving: Calories 1059; Total Fat 48g; Saturated Fat 17g;