In a medium saucepan, combine the chopped rhubarb, 1 cup water, and sugar. Place over medium heat and bring to a gentle simmer, stirring occasionally.

Let the mixture cook for about 10–15 minutes until the rhubarb breaks down completely and the liquid turns a deep pink-red color.

Remove from heat and let it cool slightly. Strain the mixture through a fine mesh sieve into a bowl, pressing gently to extract all the liquid. Discard the solids.

Allow the rhubarb syrup to cool completely. For best results, chill it in the refrigerator for 20–30 minutes.

In a large pitcher, combine the cooled rhubarb syrup, freshly squeezed lemon juice, and 3 cups cold water. Stir well until fully blended.

Taste and adjust sweetness if needed by adding honey or a little extra sugar, depending on your preference.

Add mint leaves and lightly crush them with the back of a spoon to release their aroma.

Fill serving glasses with ice, pour the rhubarb lemonade over, and garnish with a lemon slice or extra mint if desired.