Melt butter in heavy large pot over medium-high heat. Add onions and sauté until golden, about 10 minutes. Add remaining ingredients except fennel fronds. Bring mixture to boil. Reduce heat to medium-low. Cover and cook until cabbage is tender, stirring occasionally, about 1 hour.
Season with salt and pepper. (Can be made 1 day ahead. Cool slightly. Cover and chill. Rewarm over medium-low heat.)
Transfer to bowl. Garnish with re-served fennel and serve.