doesn't have to be a frost but just colder, which it generally has been - we even had frozen patches of water a few days ago
I believe you have a River Cafe recipe book, they use it a lot - cooked with pasta mainly and then there's always Ribollita and I made Bubble and Squeak with it the other night and added a chilli as well 8)
do you grow the red kale - redbor, gorgeous colour