Thinly slice the red cabbage (see note) and the onion.
In a large bowl, whisk together the mayonnaise, apple cider vinegar, sugar or honey/maple syrup) salt, and pepper.
Add the cabbage and the onion and toss to combine.
Notes
To prepare the cabbage first clean the head and cut it in half. Then cut each half through the core again. Remove the core from each quarter. Either use a knife or a mandolin to cut the cabbage into thin slices.