Heat the oil and add the onions with a pinch of salt. Saute until they turn translucent, about five minutes.
Add the tomato paste and cayenne. Cook, stirring, for about a minute or two until the tomato paste darkens.
Add the sweet potato cubes and toss well to coat with the spices. Add the white wine, enough water to cover the potatoes about halfway (I needed just over a cup), and bring to a boil.
Lower the heat to a simmer, slap on a lid, and cook for about 40 minutes or until the sweet potatoes are really tender.
Add more salt if needed. Serve hot