Let's cook.
Preheat a large non-stick skillet or griddle over medium heat.
Spread 1 tablespoon of butter evenly on one side of each bread slice. Flip two of the bread slices over so the buttered side is down.
Spread 1 teaspoon of Dijon mustard on the unbuttered side of each of the two bread slices that are butter-side down.
Layer 2 slices of ham and half of the Gruyère cheese on top of the mustard-spread bread slices. Top with the remaining two bread slices, buttered side up.
Place the sandwiches in the preheated skillet. Cook for 4-6 minutes per side, or until the bread is golden brown and the cheese is melted and bubbly. If the bread browns too quickly, reduce the heat to medium-low.
Carefully remove the sandwiches from the skillet, cut them in half, and serve immediately with a sprinkle of freshly ground black pepper.
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