In a medium-sized saucepan, stir together the rhubarb, strawberries, sugar, cornstarch, and cold water. Cook over medium heat, stirring constantly, for about 5 minutes. The mixture should thicken and no longer be milky in appearance. Transfer the hot filling to a heat-safe bowl and let cool to at least room temperature while you make the crust.

In a medium-sized mixing bowl, stir together the flour, cornstarch, and salt. Cut the cold, cubed butter into the flour mixture using a pastry cutter until the butter pieces are no bigger than peas. Add ice water, starting with 1-2 tablespoons and adding up to 3 in total or just until the dough starts to hold together when pinched. Turn onto a counter and form together into a disk with your hands. Generously flour the counter and the top of the disk of dough and begin to roll out into a circle about 12 inches in diameter. It does not to be a perfect circle, just the general shape!

Transfer the 12-inch round of dough to an ungreased 9-inch pie pan. Gently work out any air pockets on the bottom and sides. Trim the scraps from the edges and style the edge of the crust as desired. Coat the bottom and sides of the pie crust with a generous coat of flour (This helps prevent the crust from going soggy). Cover and refrigerate for 1 hour (or at least 30 minutes if you're tight on time) before assembling with the filling and baking.

Combine the oats, flour, and brown sugar in a large bowl. Stir in the melted butter to form a crumble.

Once the pie filling has cooled to room temperature, the pie crust has chilled, and the topping is made, it is time to assemble and bake the pie. Preheat the oven to 375℉.

Pour the filling into the chilled crust and smooth out the top. Sprinkle the crumble topping all over on top of the filling. Bake in the preheated oven for 45-55 minutes or until the filling is bubbly and the crust is golden. Let the pie cool for at least 2 hours before serving to allow the filling to set. Serve with whipped cream or ice cream, if desired. Enjoy!