In a medium-sized saucepan, add the diced rhubarb, sugar, salt water, and lemon zest. Cook over low-medium heat until rhubarb is soft, about 10-14 minutes. Stirring often.

Grab a bowl large enough to catch the rhubarb juice. This is important! You want to catch all that amazing flavor when straining. Use a fine-mesh strainer to keep the cooked rhubarb from becoming part of the drink.

Carefully pour the strained liquid into a freezer-friendly container. Add the fresh lemon juice and vodka. Stir.

Put the container in the freezer. Allow it to freeze before serving.

When ready to serve, add a few scoops of rhubarb slush to a glass. Top with sparkling water or club soda. Garnish with a fresh lemon slice and serve. If you want to add additional vodka, do that before serving.