Boil water in a large pot. Add green beans and a tsp of salt. Cook for 5 to 7 minutes until tender. Drain beans.
To a large bowl, add vinegar, oil, salt, pepper, garlic powder, mustard and sugar. Whisk well to combine. Add drained beans, onions and parsley. Mix well and let rest for at least 30 minutes.
Salad can be served at room temperature or cold. Chill in the refrigerator for a few hours and it tastes even better. It will keep in the fridge up to 3 days.