Follow the steps in order for the best result.
Begin by preparing the flaky pie crust. In a large mixing bowl, whisk together the all-purpose flour and fine sea salt.
Cut in the cold vegetable shortening and cubed unsalted butter using a pastry blender, your fingertips, or a food processor, until the mixture resembles coarse crumbs with some pea-sized pieces.
Gradually add the ice-cold water, one tablespoon at a time, mixing until the dough just comes together. Be careful not to overmix; the goal is a pliable dough that holds its shape.
Divide the dough in half, shape each half into a disc, wrap tightly in plastic wrap, and refrigerate for at least 30 minutes. This chilling period is crucial for a tender, easy-to-handle crust.
While the dough chills, prepare the vibrant filling. In a separate large bowl, combine the thinly sliced rhubarb, well-drained crushed pineapple, and granulated sugar. Stir gently to ensure the sugar coats the fruit evenly.
Let the fruit mixture sit for about 10-15 minutes, allowing the sugar to draw out some of the natural juices. This helps prevent a watery pie.
Preheat your oven to 425°F (220°C). Lightly grease a 9-inch pie plate.
On a lightly floured surface, roll out one disc of chilled dough into a 12-inch circle. Carefully transfer it to the prepared pie plate, allowing the edges to overhang.
Pour the fruit filling into the bottom crust, spreading it evenly. Discard any excess liquid that might have accumulated at the bottom of the fruit bowl.
Roll out the second disc of dough into another 12-inch circle. Place this top crust over the filling. Trim the edges, leaving about a 1/2-inch overhang. Fold the top crust edge under the bottom crust edge and crimp to seal, creating a decorative border.
Cut several small slits or a decorative design into the top crust to allow steam to escape during baking.
Place the pie on a baking sheet to catch any potential drips. Bake in the preheated oven for 15 minutes.
Reduce the oven temperature to 375°F (190°C) and continue baking for another 35-45 minutes, or until the crust is golden brown and the filling is bubbling thickly. If the crust begins to brown too quickly, loosely tent the edges with aluminum foil.
Once baked, carefully remove the pie from the oven and place it on a wire rack to cool completely before slicing and serving. This allows the filling to set properly, ensuring perfect slices.