How to Make It
In a small skillet, stir the curry powder over moderately high heat until fragrant, about 15 seconds; let cool. In a medium bowl, toss the apple with the lemon juice. Add the onion, mayonnaise, toasted curry powder and a pinch each of salt and pepper and toss again.
Melt 1 tablespoon of the butter in a large skillet. Set 2 slices of bread in the skillet and top each with a slice of Cheddar, 4 slices of ham, 1/3 cup of the curried apple, another slice of Cheddar and a slice of bread. Weight down the sandwiches with a medium, heavy skillet and cook over moderate heat until the bottoms are toasted and the cheese on the bottom has melted, about 5 minutes. Turn the sandwiches over, press them down and add 1 more tablespoon of butter to the skillet. Brown the second sides, about 5 minutes. Separate the sandwiches between the ham slices and add 1/2 cup of the arugula to each, then reassemble them. Transfer to plates and repeat with the remaining ingredients to make 2 more sandwiches.