Cut the chicken breasts into bite-sized chunks, put these into a mixing bowl.
Chop the garlic and rosemary leaves and mix them with half of the oil. Pour this over the chicken pieces.
Slice the onion thinly.
Heat the remaining oil in a frying pan, fry the onion for 5 minutes then add the marinated chicken. Season to taste and cook for about 8-10 minutes until the chicken turns light brown.
Meanwhile, either blanch the green beans in hot water or steam over boiling water. Drain, cover to keep warm.
Drain the butter beans and add to the chicken to warm through.
Whisk together the dressing.
In a large serving dish toss the two beans with the chicken and the dressing. Season to taste then serve.