Preheat the oven to 375℉. Arrange the unbaked tart shells on a large baking sheet and prebake according to package directions or for 10-15 minutes in the preheated oven until lightly golden. While the tart shells are baking, prepare the filling.

In a medium-sized saucepan, stir the chopped rhubarb, strawberries, sugar, and cornstarch until the rhubarb is coated. Add in the water and turn the stove on to medium heat. Cook the mixture for about 5 minutes, bringing it to a simmer, then cook on low for about another 5 minutes, stirring frequently. Turn off the heat, whisk in the butter, and let stand until ready to fill the tart shells.

When the tart shells are done baking after about 15 minutes in the oven, take them out and distribute the prepared filling evenly between all 15 shells. Place the tarts back in the oven and bake for about 15-20 more minutes or until the tart shells are golden. Let the tarts cool and or serve warm with whipped cream or ice cream. Enjoy!