Quarter, core and finely shred the cabbage. Bring a pan of salted water to the boil and blanch the cabbage for 2-3 minutes, then drain well.
Meanwhile, heat the butter in a large pan and gently cook the onion for 3-4 minutes, stirring every so often, until soft. Add the cabbage and season with salt and pepper. Cover the pan and cook over a low heat for about 8-10 minutes, stirring every