Step 1Heat the oil in a 5-quart Dutch oven over medium heat. Add the onion and cook, stirring occasionally, until tender, about 5 minutes. Add the garlic and continue to cook 1 minute. Stir in the cabbage, water, vinegar, sugar, fennel seeds and salt. Add the bacon. Bring to a boil then reduce the heat, and simmer, covered, until very tender, about 2 hours. Check the cabbage periodically and stir. If the cabbage looks dry, add a bit more water. Adjust the seasoning.
Step 2Cool thoroughly, then refrigerate the cabbage overnight. Reheat over low heat before serving.