Onion
Also known as Allium cepa, Zwiebel, oignon, cipolla, cebolla, cebula
Common in cooked, fresh and blood sausages and almost absent from dry-cured work, for a good reason: fresh onion is about ninety per cent water and full of fermentable sugar, which is the opposite of what a drying sausage needs.
Onion is everywhere in the cooked and fresh sausage traditions and almost nowhere in the dry-cured ones, and the split is not a matter of taste. A blood sausage, a liver sausage, a haggis, a white pudding or a fresh breakfast sausage may carry a substantial weight of onion, usually sweated in fat first. A salami, a coppa or a chorizo for drying will carry none, or at most a trace of dried onion powder.
The reason is what onion is made of. Fresh onion is roughly ninety per cent water, and the remainder is heavily weighted towards sugars, chiefly fructose and the fructan chains that make onion taste sweet when cooked. In a product that is going to be dried to a safe water activity, both of those are working against the maker. The water has to be driven off again, and the sugar is free food for whatever is growing, which can push a fermentation further and faster than the recipe intended or feed spoilage organisms at the surface. Raw onion also carries its own soil flora.
Cooking it first solves most of this and is why almost every traditional recipe says to sweat the onion before it goes near the meat. Sweating drives off a good part of the water, kills the surface organisms, deactivates the enzymes that produce the sharp raw pungency, and converts some of the harshness to sweetness. It also stops the onion continuing to leach liquid inside the sausage, which is a common cause of jelly pockets and a wet cut face in a cooked product. Onion added raw to a cooked sausage will usually still be perceptible as a wet spot.
The flavour chemistry runs parallel to garlic. Intact onion cells are almost odourless; cutting releases alliinase onto the sulphur precursors and produces the volatile compounds, including the lachrymatory factor that makes the eyes water. Heat destroys the enzyme, so the sequence of cutting and cooking determines how sharp the result tastes.
Dried onion, granulated or powdered, is the sensible route where a dry-cured product genuinely wants the flavour. It brings no significant water, no live flora and far less free sugar per unit of flavour. As with garlic, the substitution is not one for one by weight: dried onion is several times stronger than the fresh weight it replaces, and swapping by weight rather than by strength is how a subtle sausage becomes an onion sausage.