Melt the butter in a large pan.
Add the cabbage and stir well.
Then cover and cook over a low heat for about 15 minutes, until the cabbage is just tender but not browned.
Beat the sour cream, egg yolk, lemon juice, salt and pepper together in a cup and pour it in with the cabbage.
Bring the mixture to a gentle simmer (not a boil).
Spoon into a serving dish and serve immediately.