Wash and sterilize 10-12 mason jars and lids. Dry completely before use.
In a large stock pot over medium heat, add the sliced strawberries, chopped rhubarb, pectin, butter, orange zest, and orange juice. The mixture should condense down and slightly bubble (not boil). Stir occasionally to prevent foaming on the top.
Once the fruit is condensed down quite a bit (about 15-20 minutes of bubbling) add the sugar, 1 cup at time until fully dissolved. Continue to stir to prevent foaming on top.
Once the sugar is dissolved and the jam consistency is to your liking (I prefer very small chunks of fruit) bring the heat up to a full boil. Let the jam boil for 1-2 minutes.
Turn the stove off and let the jam rest for about 5 minutes before pouring into the jars. If there is any foam, simply remove this by scraping with a spoon or spatula.
Using a funnel, pour the jam into each mason jar with about 1/2 inch remaining at the top.
In a clean large stock pot over high heat, begin boiling enough water so the mason jars will be completely submerged. Once boiling, add the mason jars, one at a time into the water. Let them boil for 8 minutes.
Carefully take the jars out using large tongs. Place them on a cooling rack to set. The jar lids should be stable, punched down, and air tight after 20-30 minutes of cooling.
Unopened jars can be stored at room temperature. Once opened, store in the refrigerator.