Combine the chopped rhubarb, sugar, and water in a large, heavy-bottomed saucepan. Stir gently to combine.

Place the saucepan over medium heat. Bring the mixture to a boil, stirring occasionally to dissolve the sugar completely. Once boiling, reduce the heat slightly to maintain a steady simmer.

Simmer the mixture for about 20 to 25 minutes. The rhubarb will break down, and the mixture will begin to thicken. Stir frequently, especially near the end, to prevent scorching on the bottom of the pan.

Test for setting point. Drop a small spoonful of jam onto a chilled plate. If it wrinkles when pushed with your finger (or reaches 220°F / 104°C), it is ready. If not, continue simmering for another 5 minutes and test again.

Remove the pan from the heat and stir in the fresh lemon juice. The lemon juice brightens the flavor and aids in the setting process.

Carefully skim off any foam that has formed on the surface using a spoon. Ladle the hot jam into sterilized jars, leaving about 1/4 inch headspace. Seal with lids and process in a boiling water bath for 10 minutes for shelf stability, or refrigerate immediately for short-term storage.