Brassica

We've been struggling quite a while to identify this Brassica that has just come into flower in our back garden. When the leaves first appeared, we thought it might be some kind of thistle - but now the flowers confirm that it's one of the Brassicaceae. But which one, exactly? While it looks a bit like Garlic Mustard (Alliaria petiolata), the complex shape of the serrated leaves - and the rounded, Cardamine-like shape of the flowers - leave us in some doubt as to the correct identification. Also, we think it might have come from some seeds sold as a spice for cooking...