Garten makes the sandwich with white bread, shredded gruyere cheese, and butter. She grills it on a panini press before cutting it into one-inch squares to sprinkle on tomato soup.
The goal is to make the sandwich flat so that it doesn't fall apart when you cut it into croutons, so using a panini press is crucial. This also prevents the croutons from sinking.
You can also brown your sandwich in a skillet and place a large, heavy can on top as it cooks to get a thin, flat result that’s easier to cut into crouton soup toppers.
Alternatively, you can toast the sandwich on a baking sheet in the oven, flipping halfway through the cooking time until both sides are toasted and the cheese is melted.
Place another baking sheet on top as it bakes in the oven. The sandwich will come out beautifully flattened, and the cheese will act as the "glue" to hold everything together.