2 large handfuls of finely sliced white cabbage
1/2 medium onion, thinly sliced (if you have very strong onions, then you might want to reduce this down to 1/2 small onion)
1 large carrot, peeled, I then use my vegetable peeler to peel ribbon slices off the carrot as this looks nice and means you don't have to wash up a grater.
Put all of the above ingredients into a large bowl and mix well.
Season with salt and pepper.
Add the mayonnaise, I have not put a quantity here as it really depends on how much you like in your coleslaw, some people like quite a dry one, others quite creamy. Go on the basis that you add a bit at a time, as you can always add more, but cannot take out.
Transfer to your serving dish and sprinkle with a little bit more pepper, optional.