Set up a food processor with the slicing disk and process the Brussels sprouts through the feed tube. (You don't need to remove the cores of the sprouts.) Transfer to a large bowl.
In a medium bowl, whisk together the mayonnaise, whole-grain mustard, Dijon mustard, vinegar, 1 teaspoon salt, and 1/2 teaspoon pepper. Add the mayonnaise mixture to the sprouts and toss well.
Place the ciabatta halves, cut sides up, on a cutting board. Spread a thin layer of mayonnaise over each half. Place a layer of turkey on the bottom halves, pile some of the slaw on top, and place a second layer of turkey on top of the slaw. Sprinkle generously with salt and pepper. Cover with the top halves of the breads, cut sides down. Serve the individual ciabattas cut in half or cut the large ciabattas in thirds and serve.