Wash your jars and rings and lay them out on a towel to dry. Place your lids in a small saucepan and put over medium heat, so that the sealing compound softens in preparation for canning.
In a 8-quart, non-reactive pot, stir the rhubarb, strawberries and sugar together off the heat. Once the sugar has begun to dissolve, place pot on burner and bring to a boil.
Add the lemon juice/zest to the pot and let it bubble gently for approximately 15-20 minutes. As the jam cooks, use the back of a wooden spoon to mash any large pieces of fruit. Add the pectin, stir to combine and let cook for a few more minutes.
At this point, dip a spoon in the jam and see how it coats the back of the spoon. You can also try the saucer test. If you get a nice, even sheet, the jam is done. You can also taste at this point, to see if you like the balance of flavors. Add a little more lemon juice if you feel it needs additional brightening.
Pour into hot jars, wipe rims to remove any spillage and apply lids/rings.
Process in a hot water bath for ten minutes.
Remove from water and let cool. When jars are cool, remove rings and test seals. Refrigerate any jars that didn’t seal and store the rest in a cool, dark place.