Place the steamer basket in the bottom of the pot and add water until it touches the steamer. Bring to a boil. Add the potatoes, arranging them evenly on the steamer surface. Cover and cook until the potatoes are fork tender, about 20 to 30 minutes. Drain. Allow to cool until you can hold the warm potatoes without getting burned. Peel and slice the warm potatoes.
While the potatoes cook, stir together the onions and the hot broth and set aside. When finished slicing the steamed potatoes, warm the broth mixture until hot.
Put the sliced potatoes in the large bowl. Pour on the broth mixture and sprinkle on the sea salt and pepper, then add the vinegar and oil. Carefully turn the potatoes to distribute the seasonings, but try to avoid breaking the slices. Typically served warm, but you can also serve chilled.