In a large bowl, place prepared strawberries, rhubarb pieces and 1 cup sugar. Mix well. Allow the fruit and sugar to sit at room temperature for at least 6 hours or more. Overnight in the refrigerator works well too.
When you're ready to make your jam, combine all fruit, including all the juices, the remaining 2 cups of sugar and lemon zest/juice in a large saucepan over medium heat. Once the jam begins to bubble, reduce heat to medium low. Continue to cook, stirring often so it doesn't burn, until the jam has thickened to just shy of your desired consistency. This can take upwards of an hour or more. The jam will continue to set as it cools, so don't overcook it. Use the spoon or sheet test method to check gel set.
As the jam cooks, prepare your clean jars, by running them under hot water and then setting them on a dish towel near the stovetop.
Fill the jars with hot jam, leaving at least 1⁄4" of space at the top. Allow the jam to cool to room temperature, then place the lids on jars before transferring them to the refrigerator.
This recipe will make about 4 cups of jam