Combine the first 5 ingredients in a medium pot. Set over medium to medium-high heat, bring to a gentle simmer, and simmer until it thickens and the rhubarb is just tender, about 3 to 4 minutes. Cool this mixture to room temperature.
Preheat the oven to 350 degrees F. Set the tart shells on a parchment paper-lined baking sheet.
Fill the tarts with the rhubarb mixture. Bake 30 minutes, or until rims of the tarts are golden brown. Cool to room temperature. Serve as is or, if desired, top each with a little whipped cream and a mint sprig garnish.
Note: 6 to 7 medium-sized rhubarb stalks should yield the 3 cups of diced rhubarb needed in this recipe. Diced means to cut into small 1/4" to 1/2" cubes. Ready to use frozen tart shells are available in the frozen foods aisle.