Preheat oven to 375ºF.
Combine cranberry sauce, sugar and cornstarch in a medium mixing bowl. Stir in rhubarb. Pour into a pastry-lined 9-inch pie plate. Fold crust edge over the filling, pleating to fit.
Bake for 40 minutes or until golden brown. Cool completely. Sprinkle with powdered sugar before serving.
Makes 1 tart.