Wash and trim 7 cups of fresh rhubarb stalks, chop into 1-inch pieces, and place them in a large saucepan.

Add sugar and water to the saucepan with the chopped rhubarb. Stir over low heat for about 5 minutes until the sugar fully dissolves.

Increase heat to bring the mixture to a boil, then reduce and simmer for approximately 10 minutes until the rhubarb is tender.

Let the mixture cool slightly, then purée it until smooth using an immersion blender.

Transfer the puréed rhubarb mixture to a bowl and refrigerate until cold, about 1 to 2 hours.

Stir in the optional rum or substitute, mixing well.

Pour the chilled mixture into your ice cream maker and churn according to the manufacturer's instructions for 20-30 minutes.

Transfer the freshly churned sorbet into an airtight container and freeze for at least 2-4 hours for a firmer texture.