P. somniferum in the young cabbage stage, pretty obviously.
Base on how some of the leaves are yellowing, they either aren't getting enough magnesium, or they're getting rootbound (the latter happens easily which is why you need to cull the small ones, keep them spread out over a large area, etc).
I mean, or so I've heard. No idea myself.
Worst case, you can make some...delicious (🤮) poppy straw tea from the lot.