1 round loaf of crusty bread (also called a boule)
Your favorite mustard, like yellow, Dijon, coarse-ground, or honey (optional)
1/4 pound thinly sliced Swiss cheese
1/4 pound thinly sliced ham
1/4 pound thinly sliced Monterey Jack cheese
1 small Granny Smith apple
1 tablespoon vegetable oil or olive oil
4 tablespoons softened unsalted butter, divided
Preheat the oven to 350°F.
Cut the bread loaf in half horizontally. Trim the crusts from the top and bottom to make 2 large round slices that are each about 1 1/2 inches thick.
If you're into mustard, spread it to your liking on one slice of bread, then cover the surface of the bread with an even layer of Swiss cheese slices, reserving 2 slices.
Top the cheese with the ham slices, then top the ham with 2 slices of Swiss cheese and 2 slices of Monterey Jack cheese.
Quarter and core the apple, then cut into very thin slices—use a mandoline or Japanese slicer if you prefer. Layer the apple slices on top of the cheese, then finish with a layer of the remaining Monterey Jack cheese.
Spread the remaining bread slice with mustard, if desired, then close the sandwich by placing the bread mustard-side down.
Heat the oil in a heavy-bottomed, oven-safe 10- or 12-inch skillet over medium-low heat.
Spread 2 tablespoons of butter across the top of the sandwich, then place it in the skillet buttered-side down. Spread the remaining 2 tablespoons of butter on the side of the sandwich that's now facing up.
Cook for 3 to 4 minutes, until the bread is golden-brown and toasty, then flip the sandwich and cook for 2 to 3 minutes more.
Use the tip of a spatula to lift one of the bread slices and see if the cheese is fully melted; because this is such a thick sandwich, the heat may not have penetrated through to the center yet. If needed, transfer the pan to the oven and cook until the cheese is completely melted through.
Slice into 4 wedges and serve.