To prepare the roast:

• Preheat the oven with a rack in the middle position to 325 degrees F.
• Sprinkle seasoned salt evenly on all sides of the roast.
• In a Dutch oven over medium-high heat, heat the vegetable oil.
• Add the roast and brown all sides, about 2 minutes per side.
• Surround the meat with the onion wedges.
• Season the meat and onions with more seasoned salt.
• Add soft drink and beef stock.
• Bake the roast until very tender, about 3-4 hours.

To prepare the gravy:

• In a large skillet over medium heat, melt the butter. Whisk in the flour. Continue to stir occasionally, about 2 minutes. Carefully whisk in the beef stock. Add the salt and seasoning, to taste. Stir in Worcestershire sauce.
• Bring the gravy to a boil, then simmer until the gravy has thickened as desired.

To assemble the sandwich:

• Lay down a slice of bread. Top with a serving of roast beef, then top with another slice of bread.
• Cut the sandwich in half, and arrange the halves to form a V shape.
• Scoop a serving of mashed potatoes into the opening between the sandwich halves.
• Ladle gravy over the sandwich and potatoes.