Prepare roast beef as indicated on package. When heated through, shredded beef with two forks and mix with remaining au jus in the container. Set aside.
Preheat non-stick skillet over medium heat.
Spread 1/2 Tablespoon on one side of each slice of bread.
Lay 4 slices of bread, butter down, in skillet. Top each slice of bread with one slice of cheese.
Divide shredded roast beef among the four sandwiches. Top with a second slice of cheese.
Place remaining bread slices on top, butter side up.
Allow to cook on first side 3-4 minutes, until toasted.
Carefully, flip sandwich over, and cook 3-4 more minutes on second side.