In a large bowl, mix together strawberries, sugar, and rhubarb. Allow to rest for 8 hours, stirring occasionally.
Place berry mixture in a large stainless steel or enamel saucepan. On medium heat, bring to a boil. Boil rapidly for 5 minutes, stirring constantly. Add lemon juice, return to a boil, and boil rapidly for 5 minutes, stirring and breaking up strawberries to desired consistency as you stir. Remove from heat.
Meanwhile, about 20-30 minutes before processing, partially fill a water canner with hot water. Place clean canning jars in the water, cover, and bring to a boil on high heat. Boil 10 for minutes to sterilize jars.
Meanwhile, about 5 minutes before processing, place lids in boiling water and follow manufacturers instructions. (I usually use a separate smaller pot for this.)
Remove hot jars from canner. Ladle jam into jars, leaving ½ of headspace below the rim. Remove air bubbles with a clean wooden or plastic spatula and add jam to ½ headspace if needed. Wipe rim clean if needed. Center lid on jar and screw band on fingertip tight.
Place jars in canner, ensuring they are covered by about 1 inch (add more hot water if needed). Cover and return water to a boil. Once water is boiling, process half pint jars and pint jars for 5 minutes*.
Remove jars from canner and cool for 24 hours. After 24 hours, confirm that the lid is curved downwards and vacuum sealed**. Remove screw bands and dry them. Screw back loosely on the jar (or store separately).
Notes
Makes 3 cups
*High altitudes will need to add processing time:
1,000-3,000 feet add 5 minutes
3,000-6,000 feet add 10 minutes
6,000-8,000 feet add 15 minutes
8,000-10,000 feet add 20 minutes
**If any jars did not vacuum seal, refrigerate and use within 3 weeks.