Mashed Potatoes with Caramelized Onions and Cabbage

Melt half of the butter in heavy large skillet over medium heat. Add onions, cabbage and large pinch of salt. Cook until vegetables are deep brown, scraping bottom of pan frequently, about 40 minutes. Season with salt and pepper. Preheat oven to 375 degrees. Steam potatoes until tender, about 20 minutes. Transfer to large bowl. Add remaining butter and half and half and mash potatoes. Season with salt and pepper. Mix 1 cup onion mixture into potatoes. Mound potato mixture in shallow casserole. Add vinegar and broth to remaining onion mixture in skillet. Stir over high heat until liquid is almost completely evaporated, about 1 minute. Spoon onion mixture over potatoes. Bake until very hot, about 20 minutes.