Greens

Now, would anyone care to speculate as to the nature of this vegetable?

At the market it was rather helpfully labelled as 'greens'. Perhaps I should have asked, but I feared that the answer might be so self-evident as to incur disdain, and it was only a pound for a bagful. I presumed it was in season, and wasn't just some random weed they imported from Peru and decided to sell for 60p a pound. So I thought I'd give it a try. I suspect it's some kind of seed or root leaf - thick and fibrous, large white veins and tough broccoli-like stalks. But if the horticulturally-aware among you have a clue, let me know.

I'm thinking blanche and stir-fry with olive oil and garlic, which is my general approach to most vegetables of unknown aetiology.......