Heat the oil in a large nonstick skillet over medium heat. Add the onion and peppers, and sprinkle with salt and pepper. Toss to coat, and then cover and cook until almost tender, stirring occasionally, about 8 minutes. Mix in the lemon, cover and cook until the vegetables are tender, about 5 minutes. Mix in the greens beans and olives. Cook uncovered until the beans are heated through, stirring frequently. Season to taste with salt and pepper.