Lovage (Levisticum officinale)
The seed of the Lovage plant is reckoned by Grainger to be the single most common spice in Apicius. Whilst seed, root, and leaves are all edible, they are not commonly found in modern cuisine. In Roman cuisine it is the seed, typically 'pounded with pepper', which we are interested in. Despite its seeming necessity in Roman cooking, the ancient authors have little say, except Pliny the Elder who tells us that:
Lovage grows wild in the mountains of Liguria, its native country, but at the present day it is grown everywhere.- Pliny the Elder, Natural History, 19.50
Celery, celery, celery is the overwhelming flavour. When ground up, it is also the overwhelming smell. Where lovage seed differs from celery is in its bitterness, but this is not particularly overpowering. I believe Lovage seed serves the same purpose in Roman cooking as celery does in Italian. Italian cooking typically involves creating a Battuto or Soffritto, which provides a strong flavour base for the rest of the ingredients.
For something once so popular, Lovage seed is a nightmare to find. If you have a spice market or health-food shop nearby, then you might get lucky, but otherwise the answer is to hop online. Be warned, there is a similar-but-not-quite-the-same spice called ajwain which is often sold under the name Lovage.
I've recommended buying so few seeds for two reasons. Firstly, you can try the real thing, then quickly move on to the alternatives listed below. Secondly, if you're still keen to use actual Lovage seed, then it only takes a few to grow one. That's what I'm doing, following this advice. Sowing time is February/March.
Celery - Crush some in a mortar and pestle, or blitz it in a processor.
Celery Salt - Celery salt is, rather surprisingly, made from Lovage seeds!