Instructions

1. Spread 1 tablespoon of whole grain mustard on one side of each of 4 slices of rye bread.
2. Layer the thinly sliced pastrami and 2 slices of Swiss cheese on the mustard side of the bread.
3. Top with the remaining slices of bread, mustard-side facing the cheese and pastrami.
4. Preheat a panini press or griddle over medium heat.
5. Spread softened butter on the outside of each sandwich, ensuring all areas are covered for a uniform golden crust.
6. Place the sandwiches on the preheated panini press or griddle.
7. Cook until the bread is toasted to a golden brown and the cheese is perfectly melted, about 5 minutes on each side. If using a griddle, flip the sandwich halfway through the cooking time.
8. Remove the sandwiches from the heat and allow them to rest for a minute so the cheese can further meld with the pastrami.
9. Cut each sandwich in half on the diagonal and serve immediately while still warm.