Place chopped rhubarb in a medium saucepan with water.
Over medium-high heat, bring contents to a boil. Allow to boil for 10 minutes. Be sure to keep an eye on it because it may boil over. Reduce heat if needed.
Pour mixture through a strainer and into another pan. Use a spoon to press as much juice out as possible. Discard rhubarb or keep it to make a quick jam.
Pour juice back into your saucepan. Add vanilla, lemon juice and zest, and sugar.
Heat over medium-high heat and allow mixture to simmer for 10-15 minutes or until it reaches a syrup-like consistency.
Pour syrup into a sealable jar and allow to cool.
When ready to serve, pour syrup into a glass and add water until your desired taste is reached. I typically use a 1:4 ratio of syrup and water.
Notes
Fresh, firm rhubarb will give the best flavor and brightest color for the cordial.
Keep an eye on the saucepan while boiling, as rhubarb can foam and boil over quickly. Reduce heat if needed.
Press the cooked rhubarb gently through the strainer to extract extra juice without making the cordial cloudy.
The finished cordial is concentrated. For serving, start with a 1:4 ratio of cordial to water and adjust to taste.
Don’t throw away the cooked rhubarb! Use it to make a quick jam, stir into yogurt, or spoon over oatmeal.