Preheat the oven to 375˚F. Butter a 10-inch cast iron skillet (or you can use a 9- to 10-inch baking dish).
Make the topping
In a bowl, stir together the oats, oat flour, coconut sugar, cinnamon, cardamom, salt and nuts. Pour the melted butter over top and toss everything together until it’s the texture of damp sand.
Make the filling
In a large bowl, toss the pears with the lemon juice, apple cider, coconut sugar, candied ginger (if using), cinnamon and vanilla. Fold in the cornstarch.
Assemble and bake
Transfer the pear filling (and any juices that have accumulated on the bottom of the bowl) to the prepared skillet and spread them in an even layer. Scatter the pieces of butter over the pears. Sprinkle the crumble topping evenly over top.
Bake the crumble until the pears are tender when pierced with a knife and the topping is golden brown, about 45-50 minutes. Let cool slightly.
Serve the pear crumble warm or at room temperature with vanilla ice cream, whipped cream or yogurt on top!
