Slice onions as thin as possible, I like to use the 1/8 inch setting on my mandolin. Place the onions in a glass jar. In a small bowl combine white vinegar, sugar, salt, and warm water. Stir until the sugar and salt are dissolved. Pour liquid into the jar with onions. These should rest about 30 minutes for using.
Split the ciabatta loaf open, spread the horseradish sauce onto the top inside of the split loaf. Add roast beef to the bottom. Add the pickled onions, and cheese.
Place the sandwich in the press, and cook about 3 minutes or until the sandwich is heated through.
Heat the Panini press. If you do not have a panini press you may want to use a grill pan, and a heavy object to press the sandwich into the pan. Some people like to wrap a brick with foil and use this as a press.