To prepare for canning, first wash your jars and bands in hot, soapy water. Rinse them thoroughly and then place them in your water bath canner to sterilize by bringing them to a simmer. Always use new lids to ensure safe preservation.
In a pot, combine rhubarb, brown sugar, onion, vinegar, salt, and spices. Bring the mixture to a boil on high heat, stirring frequently. Reduce the heat to medium or medium-high and boil gently, continuing to stir frequently until the ingredients break down and form a thinner consistency BBQ sauce.
Use an immersion blender to blend the sauce until it's smooth, or omit this step if you prefer a chunkier sauce with pieces of rhubarb and onion.Ladle the hot sauce into the Mason jars, leaving ½-inch headspace at the top. Use a knife to slide around the inner edge of the jar to remove any air bubbles. Adjust the headspace as needed, then wipe the rims clean, place the lids on top, and screw the bands on.
Place the jars in the canner and bring the water to a boil. Process the jars in a boiling water bath for 15 minutes, adding time for your elevation if needed. After 15 minutes, turn off the heat, remove the lid, and let the jars cool in the canner for another 5 minutes. Finally, remove the jars from the canner and allow them to cool completely before storing.