Preheat oven to 425°; butter a 9-inch deep dish pie pan or cast-iron skillet or a 9-inch square baking pan.
Toss the apples, rhubarb, lemon zest, and lemon juice together in a big bowl.
Separately, rub together the brown sugar, cornstarch, cinnamon, and salt; then add to the fruit mixture; stir to combine.
Transfer the mixture to the prepared pan.
Roll out pie pastry to roughly the same size of the pan; top the apples and rhubarb with the pastry, tucking in any excess pastry between the fruit and the side of the pan.
This is a rustic dessert, so there is no need to spend any extra time making it look fancy.
Cut a few steam vents in the crust, then place the pandowdy on a baking sheet to collect any drips.
Bake for 20 minutes, then turn the oven down to 350° and bake for 25-30 minutes, or until the crust is golden and the fruit is bubbling around the edges or through the steam vents.
Cool the pandowdy for 1 hour before serving, topped with a scoop of vanilla bean ice cream.
Storage: covered with a tea towel, the pandowdy will keep at room temperature for up to 3 days.