Wash the collard greens carefully, and trim off the thick part of the midribs. Chop them coarsely and combine them with the potato dice in a large, heavy saucepan along with oil and water. Cook over low heat , covered, stirring occasionally, until the potatoes and greens are tender, about 20-30 minutes. Make sure there is always just enough water to keep everything moist, but not soupy. Add the spinach, vinegar, salt and black pepper and cayenne. Cover and steam just until the spinach wilts.
Sauté the onions in oil until they're lightly browned. Stir into the potato-green mixture. Serve immediately. Season with your favorite hot sauce.