Add the chunks along with the sugar to the saucepan (or a separate bowl).
Optionally, let the sugar and rhubarb sit for about a half hour so that the sugar draws out some of the liquid and flavor.
Add the water to the sugar and rhubarb in the saucepan and place it over medium heat.
Stirring occasionally, bring it to a boil and then turn the head down to low.
Let the mixture simmer for a few minutes. Take it off the heat as soon as the rhubarb pieces start breaking up into strings.
Cover the saucepan and allow the mixture to cool down to room temperature.
Bottle and Store
If you're going to use the lemon juice, stir it in at this point before bottling.
Add your funnel to the top of the storage bottle. Pour through a fine mesh strainer to strain out any rhubarb chunks as it goes into the funnel.
You can use the syrup immediately, or seal up the bottle and keep it in the refrigerator for 2 to 3 weeks. Discard at any sign of mold growth.
Notes
If you'd like to make this syrup keto-friendly, use the sugar substitute allulose for the best flavor and syrup thickness. You'll need to use about an extra ¼ of a cup (50 grams) for the same sweetness level.