Wash and chop greens of choice. Set aside.
In a large pan, heat olive oil. Sauté onion in olive oil until soft. Add garlic. Sauté for 1 minute.
Add greens and balsamic vinegar. Stir.
Place lid on pan and let the greens steam. Stir occasionally. Cook until greens are soft, about 5-10 minutes.
Remove from heat. Add feta cheese and pepper. Toss and serve hot.