Wash and dry the mason glass jars or freezer-friendly plastic containers with covers. Set aside.
Wash and remove the hull and stems from the strawberries and discard. Mash cleaned berries with a potato masher.
In a large mixing bowl, add 2 cups of crushed strawberries and 2 cups of white sugar. Stir and let them sit for at least 10 minutes.
In a medium saucepan, add diced rhubarb, 2 cups of white sugar, and lemon juice. Cook over medium heat until the rhubarb is softened, about 8 minutes. Let the cooked rhubarb cool down.
In a small saucepan, add the water and Sure Jell packet. Using medium heat, bring the mixture to a boil. Stir the mixture constantly for 1 minute while it is boiling. Remove from heat.
In a large bowl, add the strawberries, cooked rhubarb, and Sure Jell mixture. Stir constantly for 3 minutes. This will dissolve the sugar and create the jam.
Using a ladle and a wide funnel, fill the jars immediately. Leave at least ½ inch space on the top because the jam will expand during freezing. Add the covers and set aside.
Let the covered jam containers stand at room temperature for 24 hours. Then refrigerate for up to three weeks or freeze for up to 1 year.
If frozen, thaw in the refrigerator before using.
Notes
Note 1: Do not reduce the amount of sugar. It will cause the jam not to set up properly. Note 2: Use the original Sure Jell in the yellow box. Make sure it is not expired.