Combine rhubarb, raisin, onion, sugar OR liquid stevia, and vinegar together in a large pot. Bring to a boil and reduce to a simmer, uncovered. Let simmer for about 20 to 30 minutes till it breaks down into a coarse, relatively thick purée; you can use the back of your spoon to encourage it along.
Add the allspice, cinnamon, ginger, salt OR salt sub, and lemon juice if using. Stir well to mix.
Optional: purée to smooth in blender, put back in pot, reheat thoroughly.
Pack hot into quarter-litre (½ US pint) jars or half-litre (US pint) jars.
Leave 2 cm (½ inch) headspace.
Debubble, adjust headspace.
Wipe jar rims.
Put lids on.
Process in a water bath or steam canner.
Process either size of jar for 15 minutes; increase time as needed for your altitude.