Chop the red cabbage (the finer the better) and fill it into a big bowl.
Grate in the horseradish and apple and sprinkle with sea salt.
Mix and massage the mixture until a lot of liquid starts gathering at the bottom of the bowl or let sit for 30 minutes for the salt to do the job for you. Tip: Wear gloves for this and the following tasks if you don't want stained hands.
Start layering the mixture into clean mason jars and give it a firm push with a wooden spoon after each layer. Don't fill your jars to the very top, leave about 1-2 inch.
Use a cabbage leave or a Ziplock bag filled with water to keep the sauerkraut submerged in the liquid and close the lid.
Keep the jars at room temperature for two days, then move them to a cooler place like a basement or hallway and let it ferment for 4-6 weeks.