Might be a Nigella variety.

Cosmos?

Just throwing it out there.

I found some Nigella coming up in my french marigold seedlings.....think more cosmos for this one......worth waiting to see.....

I think this is more likely to be a wild flower, the scentless mayweed (Tripleurospermum inodorum) by the look of the growth habit. I think a Cosmos has more of a stem before getting it's feathery leaves. It could also be a stinking chamomile (Anthemis cotula) but you could tell that by gently crushing one of the leaves and having a sniff!
All the above does of course depend on how open to blown seeds your greenhouse or compost is!

I think that it's Brachycome.

Or carrot?