Slice the apple into thin ¼-inch slices and set aside. If using a cheese block, slice or shred the gouda.

Spread butter on one side of each slice of bread. On the opposite side, spread Dijon mustard if using.

Layer half the gouda and apple slices on the non-buttered side of one bread slice. Top with another slice, buttered side out. Repeat for the second sandwich.

Heat a skillet over medium-low heat. Grill each sandwich for 3-4 minutes per side, pressing down gently, until golden brown and the cheese is fully melted.

Let rest for a minute, then slice and serve immediately while warm.