To make chicken salad, combine all ingredients in a large bowl, mix well to combine, and refrigerate to cool (if not creamy enough, add more mayo and horseradish as needed to taste).
To make melts (after the chicken salad has “cooled” for at least one hour to get flavors melded), preheat a small frying pan or skillet to medium heat. Spray the pan with vegetable spray, and add one slice of bread. Top with sliced cheese, chicken salad, mozzarella, and the second slice of bread. Top the sandwich with a small piece of aluminum foil and a second pan with some sort of weight inside (like a large can of pumpkin or beans). Cook the sandwich on the first side for 2 to 3 minutes (or until lightly browned). Remove the top pan, flip the sandwich, and return the foil and second pan (with weight) to the top of the browned side. Cook on the second side (to melt the cheese and brown the bread) for 2 to 3 minutes (you may need to reduce the heat to medium-low).
Also good is a slice of tomato, which you would place between the chicken salad and mozzarella cheese.