Prepare the slaw: Whisk the mayonnaise, vinegar, mustard, sugar, caraway seeds, 1 1/2 teaspoons salt, and pepper to taste in a large bowl. Toss in the cabbage, carrot, apple and scallions; cover and refrigerate.
Prepare the fish: Whisk the egg and milk in a medium bowl; add the cod and set aside to soak. Mix the flour, cracker meal, cayenne pepper and a pinch of salt on a plate.
Heat 1 inch of oil in a heavy-bottomed skillet over medium-high heat. Remove the fish from the milk mixture and dredge in the flour mixture, turning to coat. Fry in the hot oil until golden, 2 to 4 minutes per side. Transfer to a paper-towel-lined plate to drain; season with salt and pepper. Serve with the slaw and extra mustard.