Bring a large pot of water to boil and blanch green beans for approximately 1 minute. Drain. Heat large skillet over medium heat then add the olive oil and butter. Add the onion and saute for approximately 5 minutes. Add the drained beans and continue cooking for another 10 minutes or so until the onions are carmelized and the beans are beginning to go limp. Add the balsemic vinegar for the last minute or so. Add salt and pepper to taste.