Core the cabbage and peel off the large outer leaves. Blanch them in boiling water for 5 minutes to soften. Drain and set aside.
Cut the rest of the cabbage into chunks and place the chunks in a large pan. Pour the milk over the cabbage and simmer, covered, for about 30 minutes, or until tender. Drain the cabbage and chop finely.
Meanwhile, sauté the chopped onion in the butter.