In a large, heavy bottomed, stainless steel saucepan, combine rhubarb, brown sugar, raisins, onion and vinegar. Mixture will initially get watery, then rhubarb will soften and break down and the color will darken.
Blend with an immersion blender, if desired, for a smooth sauce.
Simmer until thick. As mixture thickens, stir frequently to prevent sticking.
Add spices; cook 5 minutes longer. Ladle hot sauce into hot jars, leaving 1/4 inch headspace. Adjust two-piece caps. Process 15 minutes in a water bath canner.