I think that is a cluster of pupae of onion root maggots. I would put the leaf in a jar and keep it in the shade. Hairy little flies will emerge within a week. Then put the jar in the sun to kill them.

Thanks planatus.

It's a good image! Today I plan to scout my scallions and leeks to see if I can find somethings similar. The scallions in particular are not looking so hot, and I had a lot of root maggots in my red shallots this year.