From Apicus de re coquinaria, book II chapter V (as translated by Marco Gavio de Rubis on his web site)
Coctam alicam et tritam cum pulpa concisa et trita una cum pipere et liquamine et nucleis. Farcies intestinum et elixabis, deinde cum sale assabis et cum sinapi inferes, vel sic concisum in disco.
Cooked and pounded spelt with minced lean meat pounded with pepper, garum, and pine nuts. Stuff a casing and parboil the farcimina, then roast them with salt and serve with mustard, [whole] or cut into pieces in a plate.
3 lb pork shoulder
2 c. dried farro (see note)
1/3 c. walnuts chopped
1/4 c. fish sauce
2 tbsp freshly ground pepper or 5 cones of long pepper, ground
1.5 t. salt
Cook farrow according to package directions. Mash well (a food processor works well for this). Combine all ingredients. Let set for at least 24 hours for the flavors to mix. Stuff your sausages as per your sausage stuffer’s instructions. You do not need to make links. Parboil for 5 minutes, then bake in the oven at 350 (F) for 30 minutes.
This sausage ages very well. I froze leftovers, and after two or three weeks the sausages were better than when freshly cooked.
Notes on my substitutions:
I did not have garum, nor was I really interested in making it, so substituted a standard fish sauce you can buy in the Asian section of the grocery.
Pine nuts are very expensive, so I opted for walnuts instead. They add a very nice mouthfeel.
I did two farro blends: one at a 1:1 ratio of pork to farro (by weight, but it pretty much equates to volume) and 1:2 pork to farro. Both are equally acceptable, though a slight favor to the 1:2 version. In the 1:1 version, the fish sauce and pepper were very subtle, and perhaps need a bit more added to the mix.