Prepare rhubarb: Discard the rhubarb leaves; they are poisonous. Wash and slice the stalks (about 1 inch/ 2 ½ cm thick). Place them in the pot. 4.5 lbs/ 2 kg rhubarb Add sugar, water, and lemon juice. Stir well.
Cook rhubarb: Bring to a boil, turn the heat to medium-low, and cook the rhubarb, stirring often, for about 20 minutes or until very soft.
Blend with an immersion blender (Note 2).
Taste and add more sugar if desired. You can also add some spices to taste (Note 3).
Cook rhubarb butter: Place the pot back on the stovetop and continue cooking for about 30 minutes or until slightly thickened. Stir often, scraping the bottom and sides of the pot while you stir. Don't let the rhubarb butter scorch.
Consistency: If you like thicker rhubarb butter, cook it for more than 30 minutes until it reaches the desired consistency.
Transfer to jars: Carefully pour the preserves into jars. Use a clean cloth or paper towel to wipe any spills or excess butter from the rim of the jar. Seal the jars.
Process the jars: Place the filled and sealed jars into the canning pot with the rack, ensuring they are submerged in water. Bring the water to a boil and process the jars for 20-25 minutes.
Cool the jars: Remove the hot jars from the canning pot using the jar lifter. Place them on a clean towel or a wire rack, leaving some space between them. Let them cool undisturbed for 12 to 24 hours.
Check the seals: Once the jars are completely cool, check the seals by pressing down on the center of each lid. The jar is properly sealed if the lid is firm and doesn't flex or pop. If any jars didn’t seal, refrigerate and consume them within a few weeks.
Store in a cool, dark place away from direct sunlight or extreme temperatures (pantry or cellar). Properly sealed jars can last for up to a year.