Begin by cleaning and chopping the rhubarb into small ¼" pieces. Cut the strawberries into quarters, ensuring you remove any hulls.

In the wide pot, combine the chopped rhubarb, quartered strawberries, sugar, and bottled lemon juice. Stir gently over medium heat until the sugar dissolves and the fruit begins to release its juices, about 5–10 minutes.

Once boiling, measure the temperature and cook until it reaches 220°F, which could take anywhere from 30 to 60 minutes.

Once the jam reaches the desired temperature of 220°F, remove the pot from the heat. Skim off any foam that has formed on the surface.

With sterilized jars ready, pour the hot Strawberry Rhubarb Jam into them, leaving about ¼" headspace.

Prepare a large pot of boiling water for the water bath. Carefully submerge the jars, ensuring they are completely underwater, and cover with a lid. Process the jars in boiling water for 10 minutes.

After cooling, check the seals by pressing down on the center of each lid. Store the sealed jars in a cool, dark place.