Clean, halve and wash the beans and cook them in boiling salted water for about 12 minutes. Drain and let drain well. In the meantime peel onions and cut them into fine slices.
Heat oil and butter in a large frying pan. Sauté the onions in this pan until they are golden brown. Season with salt and pepper. Sprinkle with sugar and caramelise until golden brown. Deglaze with 150 ml water, bring to the boil and simmer for about 8 minutes.
Wash and finely chop the parsley. Arrange the beans. Spread onions over them. Sprinkle with parsley.