Roll out bottom crust, place in pie plate without stretching, trim to overhang ³/₄", and refrigerate at least 30 minutes or up to three hours.

Whisk together dry ingredients, then toss with orange zest, rhubarb and raspberries. Let sit fifteen minutes.

Roll out the top crust. Remove the pie plate from the fridge and pile the rhubarb into it. Cut the top crust into strips and weave in a lattice design over the rhubarb. Pinch the top and bottom crusts together along the edge and crimp it prettily. Brush the top crust and rim with the cold milk, then sprinkle on the fine sugar. If time allows, chill the pie in the refrigerator then the freezer for an hour or more.

Fully preheat oven to 425°. Bake for fifteen minutes then turn oven down to 375°, cover the edges with a pie shield or aluminum foil, and bake for 40–50 minutes longer. When it's done the crust will be quite brown and juice will be bubbling out.