Start by preparing your jam jars. You can do this in your dishwasher using the sterilize cycle or you can simmer the jars and lids in water on the stovetop for 15 minutes. This recipe will make 6 8-ounce jars, or 3 pint jars of jam.
Place 2 or 3 small plates in the freezer. You will use these later to test the gel point of the jam.
Combine the rhubarb, strawberries, sugar and water in a large pot. Slice the lemon in half and squeeze out all the juice into the pot. Throw the spent lemon halves and all the seeds into the pot as well - lemon peel, zest and seeds are very high in pectin, which is critical to helping your jam set. Toss these ingredients together and let them sit off the heat for an hour. This will intensify the fruit flavors.
After an hour, bring the mixture to a boil over medium-high heat. Cook the jam at a strong simmer, stirring regularly for about 15 minutes. Foam will rise to the surface. Skim this off as it appears and discard.
Reduce the heat a little to keep the jam at a constant simmer and let it go for another 15 minutes. You don't have to stir quite as regularly here, but keep your eyes on the jam and stir every once in a while to make sure you aren't scorching the bottom.
After a total of 30 minutes on the stove, check to see if the jam has set by dropping a teaspoon of jam on one of the plates that have been in the freezer. If the jam has reached its gel point it will set up quickly on the plate and won't run when you tip the plate. If it is a good consistency, proceed to the next step. If the jam is too runny, simply continue to boil it, checking every 5 minutes with a new plate.
When the jam has set, remove the lemon halves and any seeds that you can see and transfer the jam into the sterilized jars, filling to the bottom-most ring at the top of the jar. If you plan to consume the jam within a couple of weeks, top the jars with lids, secure with the rings and store in the refrigerator. If you want to can the jam, proceed with the next step.
Once you've filled the jars, tap the jars on the countertop to release any air bubbles and wipe the rims of the jars clean. Top the jars with the lids and secure with the rings. Bring a large pot of water to a boil and place a rack in the bottom of the pot. Lower the jars into the boiling water to rest on the rack. Boil the jars for 5 to 10 minutes and then remove the jars carefully and let them cool on the countertop. You will hear a pop for each jar as the jam cools - this is a sign that the seal has formed on the lid. If the jars have sealed properly (check by removing the ring and making sure the lid is secure), you can store this jam at room temperature for up to a year.