Preheat oven to 425°. Put cranberries, raisins, apples, and 3/4 cup water in a medium saucepan. Heat over high heat until boiling and fruit starts to soften, about 4 minutes.
Mix cornstarch with 1/4 cup cold water. Stir into boiling fruit; mixture will thicken slightly. Remove from heat and add butter, zest, juice, spices, and 3/4 cup sugar.
Line a 9-in. pie pan with one of the crusts. Sprinkle crust evenly with walnuts, pressing gently so they adhere to dough. Fill crust with fruit mixture. Lay second crust over the top. Crimp edges to seal and cut a few decorative steam holes in top crust. Brush with milk and sprinkle with remaining 2 tsp. sugar.
Set pie on a foil-lined baking sheet and put on bottom rack of oven. Bake 10 minutes, then reduce oven temperature to 375°. Bake until golden brown and juices are bubbling around edge, 35 to 45 minutes total (check halfway through and cover rim with foil if crust is getting too dark). Let cool 30 minutes before cutting. Serve with whipped cream.