With its milder and sweeter flavour, leeks are often used in soups and stews as a substitute for onions. Approximately 350 seeds / gram.
Hints:

Can be harvested early and used like a green onion.

Start indoors and transplant 15 cm apart in rows 60 cm apart.

American Flag - 112-140 days. Leaves are medium green. The stem is fairly long and best harvested at about 5 cm [2"] thick. Quite hardy.