Preheat your oven to around 220 ° C on fan bake and turn on the extractor fan (there will likely be a fair bit of smoke when you open the oven door during the roast).
Spread green beans evenly over a roasting tray. Make sure that the beans are in a single layer, more crowded on the perimeter and more spaced out in the centre.
Place the tray in the oven (only once it's reached temperature).
When the beans start turning yellow, mix the beans moving those on the outside to the inside and vice versa.
Watch carefully and at first crack turn the beans again (as per previous step). As the beans complete first crack, turn them again.
At this point, do not leave the beans. Turn them again as second crack starts.
Take the beans out of the oven when they've reached the desired roast. Take them outside to cool them, blowing away the chaff as you agitate them. There may be some chaff in the oven which should be vacuumed out when the oven has cooled.