Melt the tablespoon butter in a skillet over medium heat. Sprinkle half of the packet of dry onion soup mix on the onions. Saute the onions on until they are soft and translucent, stirring frequently. Reduce heat to medium low and allow the onions to caramelize until they are deep brown in color.

To assemble, spread one side of each piece of loaf bread with butter. Sprinkle a small amount of the remaining onion soup mix on the buttered side of the bread slices. Build the sandwich by layering the bread on bottom, a slice of cheese, then the caramelized onions, another slice of cheese & finally a second slice of bread, butter side facing out. Put the sandwich butter-side down on a griddle or skillet over medium-low heat. Grill the sandwich until one side of the bread is golden and slightly crisp. Flip the sandwich over to grill the other side. While the sandwich is grilling, add another layer of onions & another slice of cheese, then top with the third slice of bread. Flip the sandwich one last time to grill the final slice of bread and continue cooking until the bread is golden & the filling is warm/cheese is melted.

Once the sandwich is cooked, cut off all 4 edges of the bread. Cut the sandwich into quarters & secure with toothpicks if desired.