Wash leafy green vegetables thoroughly. Drain and chop into bite size pieces. Heat olive oil in pan and saute leek 3-4 minutes. Toss in vegetables. Add a few tablespoons of water or vegetable broth and cover the pan. Cook until just tender, but still bright green. Depending on the type of green vegetable you choose, this can take about 4-8 minutes. Just keep checking to make sure you don't overcook. Add splash of vinegar and serve.