Preheat oven to 230ºC . Make the pre-baked pie crust. In a bowl combine the rhubarb, strawberries, cornstarch, water or juice, and sugar. Mix well to coat the fruit with cornstarch and sugar, then let sit for 10 minutes. Fill the pre-baked pie shell with the fruit mixture. Cover the edges of the crust with foil to prevent browning. Bake for 10 mintues at 230ºC , then reduce heat to 180ºC , and continue baking for 25 to 30 minutes. Remove the pie from the oven, and let it cool to room temperature before serving.