Prepare canning jars, lids, and heat a water bath canner.
Wash rhubarb and peel if desired. Remove the tops and bottoms of the stalks and discard any leaves. Chop rhubarb stalks into ½ inch to 1 inch pieces.
Mix the rhubarb and sugar and allow it to macerate for at least an hour (or overnight) to release it's juices.
Whisk the clear gel into the measured cold water (or juice) called for in the recipe. Make sure it's completely dissolved, then set aside.
Add the macerated rhubarb to a large stock pot and bring to a gentle simmer.
Next, add in the dissolved clear gel and water mixture and cook until thickened.
When the mixture has thickened and begun bubbling, add the lemon juice and boil for one minute more.
Remove from heat and quickly ladle in prepared canning jars, leaving 1 inch headspace. Be sure to de-bubble the thick liquid as well.
Wipe rims to ensure a tight seal and apply 2-part canning lids, twisting until fingertight. Process in a water bath canner for 30 minutes, increasing time as needed for altitude.
After the processing time is complete, turn off the water bath canner and allow jars to rest for another 10 minutes to reduce the chances of siphoning.
Remove with the aid of a jar lifter and place atop a towel-covered surface to cool. Check seals after 12 to 24 hours. Any unsealed pie fillings should be moved to the refrigerator to be used first.
Store properly sealed jars in a cool dark location. Refrigerate after opening. Jars of rhubarb pie filling will maintain peak quality for 12 months when properly canned and stored.