Combine the rhubarb, honey, water, and lemon in a sauce pan. Place over high heat and bring to a bubble. Reduce the heat to medium-low and cook at a bare simmer for 15 minutes, or until the rhubarb has fallen to bits and the lemon is quite soft.

Let the cooked rhubarb and its liquid cool for a few minutes. Then, position a fine mesh sieve over a medium-sized mixing bowl and strain out the cordial. Work the pulp with a spoon or spatula in order to release as much liquid as possible.

Transfer finished cordial to a jar and refrigerate. It will keep 3-4 weeks in the fridge.

To can, funnel into prepared jars and process in a boiling water bath canner for ten minutes.