Combine the brussels sprouts, apple, and shallot and toss with the dressing. Sprinkle with salt and plenty of black pepper and refrigerate until ready to serve. (It’s best to let the slaw rest for an hour or so to allow the flavors to mellow; the sprouts will also soften a bit and exude some juice. You can let it sit longer, for up to 24 hours, if you like. Drain the slaw before continuing.) Just before serving, toss with the mint.