Clean, halve and wash the beans and cook them in boiling salted water for about 12 minutes. Pour into a sieve and drain well. In the meantime peel onions and cut them into fine slices.
Heat the oil and butter in a large pan, fry the onions in it while turning them well (lightly brown). Season with salt and pepper. Sprinkle with sugar, let it caramelize, add 150 ml water, bring to the boil and let it simmer for 6-8 minutes.
Wash parsley, shake dry and chop finely. Mix the beans briefly with the onions, heat up and arrange on a plate sprinkled with parsley.