"K" wrote

The blades are the cotyledons or 'seed leaves' - they provide the first
bit of food while the seedling gets established. There should be two of
them on each seedling. There will be a pause while the seedling gets a few
roots going, then it will produce the first true leaves, which are fine
and feathery.

Most of the carrot family seem to produce long thin cotyledons.

--
Thanks Kay

I'll be patient for a while and not replace them with onions immediately.

Phil