1. Trim the green beans, wash them, and cook them for about 15 minutes in a large pot of salted boiling water, uncovered.
2. Meanwhile, peel the onions, wash them, and cut them into thick slices.
3. Gently sauté the onions in butter in a sauté pan for 5 to 6 minutes, without letting them brown.
4. As soon as the beans are cooked, drain them in a colander. Immediately refresh them and sauté them with the onions for 3 to 4 minutes.
5. Sprinkle with chopped parsley and serve immediately.