Wash the head of cabbage under cold running water, remove the tough outer leaves, and cut cabbage into quarters. To shred the cabbage, cut out the core and slice the quarters crosswise into 1/8?inch-wide strips.
In a heavy 4 to 5-quart casserole, heat the oil over moderate heat. Add chopped onions and cook, stirring frequently for 5 minutes, or until the onions are lightly browned. Add the cabbage, stir thoroughly, add the apples, and pour in the boiling water. Bring to a boil over high heat, stirring occasionally, and reduce the heat to very low. Add red wine vinegar, salt and sugar, stir to combine. Cover and simmer for 1 1/2 to 2 hours, or until the cabbage is tender. Check from time to time to make sure that the cabbage is moist and tender. If it seems dry, add a tablespoon of boiling water. When the cabbage is done, there should be almost no liquid left in the casserole. Taste for seasoning, then transfer the entire contents of the casserole to a heated platter or bowl and serve.