In a large, heavy bottom saucepan or dutch ovwn, mix the 6 pints strawberries and 4 stalks rhubarb with the 2 cups sugar. Stir in the ½ lemon juice and 2-3 tablespoons water. Mix to combine.
Place a plate in the freezer. Bring the mixture to a boil over medium heat. Reduce heat and simmer for about 30 minutes; you want the jam to reduce and thicken before removing from the heat. Take the plate from the freezer and add a spoonful of jam; if it is set and not drippy, the jam is done. If it's still liquidy, keep cooking the jam and test it again.
When the jam has thickened, pour into prepared sterilized glass jars. Can the jam or keep in the fridge for 2 weeks. Jam will further thicken as it cools.
Notes
CN's Jam Tips
Add more sugar. Rhubarb is finicky, and I love it for its slightly sour, curmudgeonly vibe. However, it may be too tart for some. You can add more sugar to taste if you find it's still too mouth-puckery after cooking.
Use ripe strawberries. This will help the jam to turn out nice and sweet without a ton of added sugar. If the strawberries are not super ripe, you may need to add more sugar.
Try frozen fruit. If you're like me and really love rhubarb, you may keep a stash in the freezer so you can still use it when rhubarb season is over (same goes for strawberries). You can make this jam using frozen rhubarb and strawberries if needed. Tip: Chop up the rhubarb before freezing it so it's easy to work with later. There's no need to defrost either the rhubarb or the strawberries before you start making the jam.
The plate test. Place a plate in the freezer to chill while the jam cooks. When the jam coats the back of a wooden spoon, drizzle a spoonful of jam on the cold plate. If the jam "wrinkles" and doesn't run, it's set and ready to take off the heat. If it's still runny, keep cooking it and try again in a few minutes.