Wash and stem the kale, and chop the stems in small pieces. Reserve. Melt the butter or heat the oil in a large pot, then add the kale stems, onion and potato slices. Sauté over medium heat for about 10 minutes, until the stems and potato slices are soft when pierced with a knife, and the onions are translucent. Add the garlic and the kale leaves and stir. Cover the pot and reduce the heat to low, and let the kale steam for about 5 minutes. Uncover and stir.