Begin by selecting a bread with a tight crumb to ensure a crispy exterior and a non-pasty interior. Slice the bread to your desired thickness.
Shred the Gruyere cheese after removing the rind. Avoid using pre-shredded cheese for better melting quality.
Heat a non-stick pan over medium-low heat. Spread mayonnaise evenly on one side of each bread slice for a golden, crispy finish.
Place a slice of bread, mayonnaise side down, in the pan. Layer with American cheese and shredded Gruyere cheese, ensuring the cheese reaches the edges.
Season the cheese with a pinch of salt and pepper to enhance the flavors. Top with another slice of bread, mayonnaise side up.
Cook slowly over medium-low heat, allowing the bread to crisp and the cheese to melt thoroughly. Check occasionally to prevent burning.
Once the bottom is golden brown, carefully flip the sandwich and continue cooking until both sides are crispy and the cheese is fully melted.
Remove from the pan, slice in half, and serve immediately. Enjoy the gooey, cheesy goodness with a side of crunchy potato chips if desired.