Roll out 1 double pie crust for a 9" pie on a floured surface.
Line a 9” pie pan with half of the rolled out crust. Lightly poke the bottom crust with a fork.
In a large mixing bowl, combine the 3 cups fresh raspberries, 3 cups fresh or frozen chopped rhubarb, (see notes below if using frozen rhubarb), 1 3/4 cups granulated sugar, 3 tablespoons cornstarch, 1 pinch of salt, and 1 teaspoon fresh lemon zest. Stir until the fruit is completely coated.
Pour the filling into the prepared pie plate; dot with 3 tablespoons unsalted butter, cut into pieces and top with remaining rolled out crust. Seal, then flute the edges.
Cut several slits in the top crust with a knife to allow steam to escape while baking. Brush top crust with half and half or milk, then sprinkle with additional granulated sugar.
Place pie on a lined baking sheet and use a pie shield or foil to protect edges of crust while it bakes. Bake pie at 375°F for 30 minutes, then remove foil or pie shield and bake an additional 25 minutes, or until crust is golden brown.
Allow pie to cool completely before serving.