Let's cook.
In a small bowl, combine the mayonnaise and Dijon mustard; mix well to create the spread.
Spread a thin layer of the mayonnaise-mustard mixture on one side of each of the 4 bread slices.
Place 2 slices of bread, spread-side down, on a clean surface. Layer 2 slices of turkey breast on each, followed by 1 slice of Swiss cheese.
Top with the remaining 2 slices of bread, spread-side up, to form two sandwiches.
Melt the butter in a large skillet over medium heat. Once melted, carefully place the sandwiches in the skillet.
Cook for 3-4 minutes per side, or until the bread is golden brown and the cheese is melted and bubbly.
Remove from the skillet, slice in half if desired, and serve immediately.
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