1Put the red cabbage in a small glass or ceramic bowl. Drizzle with half the vinegar and season with salt and pepper. Toss well. Repeat with the green cabbage and remaining vinegar in a separate bowl. Let both marinate for 15 minutes. Drain.
2Preheat the sandwich press until the READY light illuminates.
3Butter the bread slices and place 6 slices buttered side down.
4Spread 6 slices with the horseradish sauce. Top with half the cheese slices.
5Top with the corned beef and drained red and green cabbage. Top with the remaining cheese and sandwich with the remaining bread slices, buttered-side facing up.
6Place two of the sandwiches onto the sandwich press and lower the lid. Cook for 5 minutes until the bread is toasted and the filling is heated through.
7Remove the sandwiches using a silicone spatula. Cover to keep warm. Repeat with remaining sandwiches.
8Cut in half and serve with the pickled onions and herbs.