In a medium saucepan, combine the chopped rhubarb, granulated sugar, and water.

Place the saucepan over medium heat and bring the mixture to a simmer, stirring occasionally.

Once the mixture begins to simmer, reduce the heat to low and let it cook gently for about 10-15 minutes, or until the rhubarb is completely softened.

Remove the saucepan from heat and let the rhubarb mixture cool slightly.

Once the mixture has cooled, transfer it to a blender or food processor and puree until smooth.

Strain the rhubarb puree through a fine mesh sieve into a bowl to remove any fibrous bits.

Stir in the fresh lemon juice until well combined.

Cover the bowl with plastic wrap and refrigerate the rhubarb mixture until completely chilled, preferably for several hours or overnight.

Once the mixture is chilled, pour it into an ice cream maker and churn according to the manufacturer's instructions until it reaches a smooth and creamy consistency.

Transfer the churned sorbet to a freezer-safe container and freeze for a few hours to firm up before serving.