Place rack in lowest position in the oven. Preheat the oven to 375 degrees F (190 degrees C). Line a 9-inch pie plate with pie crust.

Combine apples and rhubarb in a large bowl. Toss with sugar, 3 tablespoons plus 1 teaspoon flour, and Chinese five-spice. Pour filling over the crust.

Combine unsifted flour, sugar, oats, salt, and five-spice together in a large bowl. Cut in butter until crumbly. Scatter crumb topping over the pie.

Bake in the preheated oven until topping is golden brown and filling is bubbly, 50 to 60 minutes. Let cool completely before serving.