This is a cream-stewed cabbage dish that we in my family have at 2nd day of Christmas (the 26th of December). we eat it with glazed ham, sugar glazed potatoes and mustard. We eat this more as a primary item at the meal, than as a sauce.
Melt the margarine or butter and whisk in the flour, then add the milk or cream a little at the time. Let it cook for a short time while you are whisking til it's thick enough to cover the back of a spoon.
Then add the cabbage and let it simmer for about 10 minutes. It has to be rather thick and a nice dark-green color (not a white sauce with cabbage pieces in it, then it is too thin and you need to make it a bit thicker).