In a mixing bowl, combine the thinly sliced red onions, white vinegar, sugar, salt, and warm water. Stir well until the sugar and salt dissolve. Let the onions sit for at least 15 minutes to pickle.
Slice your ciabatta bread in half, spread horseradish sauce on both halves, layer sliced roast beef on the bottom half, and top with two slices of white cheddar cheese.
Once the onions are pickled, drain them and sprinkle generously over the cheese. Place the top half of the ciabatta on the sandwich.
Preheat your panini press or skillet over medium heat. Spray cooking spray on the skillet. Cook the assembled panini until golden brown and cheese has melted, about 3-5 minutes per side.
Remove the panini from the heat, let it sit for a minute, then slice in half and serve immediately.
