Chop the onion and mince the garlic. Coarsely chop the mustard greens. Add oil to a heated pan and sauté the onions. Add garlic once onions are transparent to slightly brown. Sauté garlic for about one minute (do not let it turn brown). Add mustard greens corn and black beans. Cover the pan and let it cook for about 2 minute on medium heat. The greens will reduce in size and easier to mix in the pan. Add salt and mix all ingredients. Cook for 5-10 minutes. Turn off the heat, add freshly squeezed lime or lemon juice according to taste. Serve and top it with avocado or any fruit such as pear, apple or berries in season.