01 - Spread butter evenly on one side of each bread slice. Place one cheese slice on the unbuttered side of two bread slices, then top with the remaining bread, buttered side facing out.
02 - Heat a skillet over medium heat. Add assembled sandwiches and grill 2–3 minutes per side until bread is golden and cheese is melted.
03 - Pour tomato soup into a saucepan. Add water or milk according to can instructions. Heat over medium, stirring occasionally, until hot but not boiling.
04 - Cut grilled cheese sandwiches as desired. Serve immediately with bowls of hot tomato soup for dipping.