I agree it's definitely a member of the brassica family.
Did you plant kale last year? The leaves are quite large, from what I can see.
I thought I had planted kale but never found any that had survived transplant (or so I thought). I normally plant curly kale, and I had none of that last year. It might be a new variety I decided to try out, not looking like my normal perception of what kale looks like, I completely overlooked it.
Maybe it is worth picking the leaves and steaming them to try it out, before it seeds and dies.