Cut the onion into thin slices. Add to a glass, heat-proof jar. Set aside.
In a small saucepan placed over medium-low heat, combine the vinegar, water, and sugar. Stir to dissolve the sugar. Heat for 3-4 minutes. Pour the liquid over the onion slices. Let cool. Refrigerate for up to one week.
In a small bowl, stir together the mayonnaise and horseradish. Season to taste with salt and pepper. Set aside.
Slice the ciabatta rolls in half. Spread horseradish mayo on each half.
On the bottom, layer pickled onions, roast beef, more onions, and a slice of cheese. Replace the top half of the roll.
Heat a grill pan or skillet over medium heat. Place the sandwiches in the pan and place a bacon press (or cast iron pan) on top to compress the sandwiches.
Cook 2-3 minutes until brown and flip over. Cook another 2-3 minutes until brown and warmed through.
The pickled onions and horseradish mayo can be made ahead of time. Both will keep well for several days in the refrigerator.
Use thinly sliced beef to create a tender, easy-to-eat sandwich.
Assemble the sandwiches right before toasting and serve immediately.
You don't need a panini press to make this sandwich. A skillet works just fine. Use a bacon press (or heavy cast-iron pan) to weigh the sandwich down while each side toasts.