Pieris napi Green-veined White
This butterfly along with the other whites usually flits quickly around the garden flowers rarely settling for long although plenty visit the garden each year. On the slopes of this mountain though, it was quite cool and I managed to get close enough to take some sort of picture.
This underside view shows how similar it is at first glance to a Wood White with which it is often confused.
It has a variety of food plants in the wild such as Sinapis arvensis (Charlock) and various Cardamine ssp but I remember many caterpillars on our Nasturtiums in my parents' garden.