Slice the red cabbage into ½ inch strips. In a one liter mason jar (or other similar sized container) mix together water, vinegar, salt, and sugar until salt and sugar are mostly dissolved. Add cabbage and garlic, making sure that all of the cabbage is fully covered with liquid. Refrigerate for at least four hours.
Combine all ingredients in a small bowl, mixing to combine.
Recipe makes four sandwiches. Instructions are for one sandwich: Preheat panini press to medium high heat*. Remove ½ cup pickled cabbage from container. Pat the cabbage with a paper towel to remove excess liquid.
Spread two tablespoons of horseradish sauce on two slices of rye bread. Layer on one piece of rye bread: ½ roma tomato (sliced), ¼ avocado (sliced), ½ cup spinach, one slice of Swiss cheese, ½ cup pickled cabbage. Press on panini press until golden brown, about 3-5 minutes. Enjoy immediately.
*If you don't have a panini press, you can make this sandwich on the stovetop. Heat a medium frying pan over medium high heat; layer with thin coat of oil if needed. Place a plate on the sandwich and press down, as a panini press would. Flip when sandwich is golden brown.