Using a microwave-safe plate or bowl, add the onions and cook in microwave until tender, about 4-5 minutes.
Coat a large nonstick pan with cooking spray. Add onions, ½ cup Guinness beer, and Worcestershire sauce. Simmer for about 15 minutes until the liquid has mostly evaporated. Stir often. Remove from heat and mix in mustard.
Spread the outside of each slice of bread with ½ teaspoon reduced-fat butter. Add ½ the roast beef (2 ounces) on the inside of one slice of bread. Do the same for the other slice. Top each with half of the cooked onions and the cheese slices. Finally, top each with the other slice of bread, buttered side up.
To Cook in Panini Maker: Heat the panini maker or George Foreman Grill. Add both sandwiches and cook for about 4 minutes until golden brown and cheese have melted.
To Cook in Pan: Coat a nonstick pan with cooking spray. Add both sandwiches and cook for about 2-4 minutes on each side until golden brown and cheese have melted.