# Step-by-Step Instructions:
01 - Arrange four bread slices on a clean surface.
02 - Spread a thin, even layer of mayonnaise on one side of each bread slice.
03 - If desired, spread a thin layer of Dijon mustard on the opposite side of two bread slices.
04 - Place two slices of cheese between two bread slices, ensuring mayonnaise-coated sides face outward; repeat for second sandwich.
05 - Preheat a nonstick skillet over medium-low heat.
06 - Place sandwiches in skillet mayo-side down; cook for 3 to 4 minutes until golden brown.
07 - Flip sandwiches and cook other side 3 to 4 minutes, pressing gently with a spatula until bread is golden and cheese melts.
08 - Remove from skillet, let rest for 1 minute, then slice and serve immediately.