Combine the cabbage, fennel, carrots, apples and onion in a large bowl. Douse with the vinegar, season with salt and let sit at least 1 hour. This will soften the cabbage and make it seem almost cooked, but it will still have a great texture.
Drain the cabbage of excess liquid but still keep it fairly juicy. Stir in the mayonnaise and horseradish. Taste for seasoning and add salt. The result should be a creamy and tart slaw.