• Chop rhubarb into rough dice and set aside.
• Thinly slice the onions and place in a separate bowl. Cover the onions with boiling water and let stand for 5 minutes. Drain and discard water.
• In a heavy-bottomed pot, dissolve sugar in cider vinegar over medium heat, stirring to ensure that the sugar doesn’t burn. Add the onions, rhubarb, salt, ground cloves, cinnamon, allspice and cayenne pepper.
• Stir well. Bring to boil, reduce heat and simmer uncovered, stirring often, until it reaches a thick jam-like consistency. This should take about 1 hour. Remove from heat.

Prepping The Jars

• While the rhubarb is simmering, prepare your canning jars and lids according to standard canning procedures. I like to wash the jars, lids, and bands in the dishwasher ahead of time. I then simmer the jars for a few minutes in the canner in water and then keep hot in the oven at 200F.
• Once the rhubarb is cooked, remove the pot from the heat. Using a soup ladle, spoon the rhubarb mixture into the hot sterilized jars, leaving about 1/2 inch of headspace.
• Wipe the jar rims with a clean, damp cloth or paper towel to remove any syrup or residue from the top of the jars. Place the lids on the jars and screw on the bands until fingertip tight only.

Canning the Rhubarb

• Process the jars by placing them on a rack inside a pot large enough to cover the tops of all jars with boiling water. Process for 15 minutes, adjusting for altitude if necessary.
• After processing, carefully remove the jars from the water bath and let them cool completely on a towel-lined countertop.
• Once cooled, check the seals, ensure the top jar button is depressed, and label the jars with the date. Store them in a cool, dark place for up to six months. You will hear the jars pop as they are cooling and sealing.