Prepare the rhubarb by cutting off and discarding the leaves and trimming both ends. Never eat rhubarb leaves, they are not safe to eat. Rhubarb should be washed and stripped of any tough, stringy ribs using a paring knife. Forced rhubarb does not need peeling, generally. Thinly slice the rhubarb once it's prepared. You can also use frozen rhubarb, which may already come sliced.
11 ounces Rhubarb
Remove the stems and roughly chop strawberries. You can use fresh or frozen strawberries.
11 ounces Strawberries
Once the fruits are prepared, add them to a large, heavy-bottom saucepan. Pour the sugar over the fruit and stir.
11 ounces Granulated sugar
At this point, you can begin the jam-making process, or refrigerate and let the jam develop flavors for up to 24 hours in the refrigerator. This generally will give you better results, but it isn't necessary. Read on for further instructions.
Add your prepared fruit to a heavy-bottom large saucepan. Add lemon juice. Cook gently over medium heat for 10 minutes to allow the sugar to dissolve and the fruit to break down, if you didn't store it overnight. If you stored it overnight in the refrigerator, it will already be broken down with the sugar dissolved, in which case you should move on to step 5 right away.
1 Tablespoon Lemon juice
Raise the temperature to medium-high heat and let the mixture gently boil for 10-15 minutes until the temperature reaches 221°F on an instant-read thermometer or candy thermometer. This is the jam setting point, so don't take it off the heat until it reaches this temperature.
Take the jam off the heat and let it cool completely before storing. When ready to store, ladle it into airtight, clean jars, and store in the refrigerator for up to 3 weeks.