Cut the cabbage into 1-inch slices and separate the leaves into pieces. Place the cabbage in a heavy skillet with the water and cook, covered, over medium heat for 20 minutes, or until the cabbage is fork-tender. While the cabbage is cooking, melt the butter in a second skillet and saute the onions until they are translucent. Add the flour and the curry powder, stirring to make sure they are well mixed. Cook for 2 to 3 minutes, then add the milk and continue to cook until the sauce is smooth. When the cabbage is cooked, drain it and place it in a serving dish. Pour the curry sauce over it and serve hot.