Place a plate in the freezer. Combine 2 pounds rhubarb, 2 cups granulated sugar, ¾ cup cold water and juice of 1 lemon in a saucepan or Dutch oven set over high heat and bring to a boil. Reduce heat to medium-low and cook, stirring occasionally, until thick, about 45 minutes. Jam will continue to thicken as it cools.
Notes
CN's Jam Tips
Add more sugar. Rhubarb is obviously quite tart, so I don't like to skimp on the sugar. However, you don't want to make it overly sweet. I recommend tasting as you go and carefully adjusting/adding more sugar until it reaches your desired sweetness.
Mix in some honey. I love swapping a bit of the sugar for honey, which beautifully complements the floral rhubarb. Add 1 cup of honey in place of the honey if you want to try this out.
Use very 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.
Use the plate test. Place a plate in the freezer to chill while the jam cooks. When the jam looks like it's done (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 pretty loose, keep cooking the jam and try again in a few minutes.
Add salt. I learned this trick from a comment on a NYT recipe for apple jelly and have started incorporating it when I make jam. A big pinch of fine sea salt stirred into the jam pot will accent its sweetness.