To prepare crust, lightly spoon flour into dry measuring cups; level with a knife. Combine flour, 3 tablespoons sugar, and salt in a large bowl; cut in butter and shortening with a pastry blender or 2 knives until mixture resembles coarse meal. Sprinkle surface with ice water, 1 tablespoon at a time; toss with a fork until moist and crumbly (do not form a ball).
Gently press two-thirds of dough into a 4-inch circle on heavy-duty plastic wrap, and cover with additional plastic wrap. Roll dough into a 12-inch circle. Press remaining dough into a 4-inch circle on heavy-duty plastic wrap, and cover with additional plastic wrap. Roll dough into a 9-inch circle. Freeze both dough portions 10 minutes or until plastic wrap can be easily removed.
Preheat oven to 425°.
Remove 1 sheet of plastic wrap from 12-inch dough circle; fit dough, plastic-wrap side up, into a 9-inch pie plate coated with cooking spray, allowing dough to extend over edge of plate. Remove top sheet of plastic wrap; fold edges under, and flute.
To prepare filling, combine rhubarb and the next 4 ingredients (rhubarb through nutmeg), tossing well to combine. Spoon filling into crust.
Remove top sheet of plastic wrap from remaining dough. Cut dough into 12 (3/4-inch) strips. Gently remove dough strips from bottom sheet of plastic wrap, and arrange in a lattice design over rhubarb mixture. Seal dough strips to edge of crust. Brush top and edges of dough with milk; sprinkle with 1 tablespoon sugar. Place pie on a baking sheet covered with foil. Bake at 425° for 15 minutes. Reduce oven temperature to 375° (do not remove pie from oven). Bake an additional 30 minutes or until golden (shield crust with foil if it gets too brown). Cool on a wire rack.