Spread a small amount of mustard on each slice of beef, top with a slice of bacon, a few slices of onion and a a pickle spear half. Roll up and secure with toothpicks. Heat oil in a skillet over medium heat, add rolled beef slices and brown on all sides. Remove beef rolls to a place and set aside. In same skillet add butter; stir until melted. Add beef stock, wine if using and bring to a boil, stirring constantly, reduce heat to a simmer. Stir in cornstarch and season with salt and pepper. Return beef rolls back to the skillet and let simmer for 45 minutes or until heated through.