Begin by generously seasoning your steak with coarse sea salt and placing in the fridge to dry brine overnight. If you do not have that much time, season and place in the fridge for up to 40 minutes.
Add all the ingredients for the horseradish spread to a bowl, mix and place in the fridge until ready to serve
Thinly slice your red onions.
Then heat up a cast iron skillet to low heat.
Melt your butter, then add the onions and slowly caramelize until thickened, dark red and sticking together (about 40-60 minutes).
About 10 minutes before ready, add your red wine and let simmer into the onions.
Pull off once done and keep warm until ready to serve.
Preheat your oven to 850F for indirect cooking (you can easily do this in your broiler or salamander grill as well!).
Pull your steaks out and add them to a high temperature baking sheet and place in the oven to cook for 2 minutes making sure to rotate the steaks halfway through.
Then, flip the steaks over and cook for another 2 minutes until they are sizzling and have developed a nice crust.
When they are 120F internal, pull off and let cool for 8 minutes.
Finally, thinly slice your steak and start building your sandwich.
Add some horseradish spread to your toasted bread topped with sliced steak, onions, cheese (make sure to melt), more chopped chives and more sauce if you wish. Serve and enjoy!