In a small bowl mix mayo and horseradish, set aside.

Assemble sandwiches: On one side of bread, spread some butter, flip slice over and add 1 slice Havarti, 4-5 tomato slices, 1 slice of cheddar. Then smear the horseradish spread over one side of the remaining bread slice, and close the sandwich with the spread facing the cheese. Spread some more butter on the top of the slice. Repeat for second sandwich.

Heat a large skillet over medium heat. Add both sandwiches to the skillet, and cook for 3-4 minutes on the first side (watch closely for it to turn golden brown), then carefully flip sandwiches, and cook for 2-3 more minutes on the second side (this side will cook faster).

Remove from skillet and cut in half. Serve immediately.