Preheat the oven to 350°F (175°C) and butter a 9-10 inch skillet.

In a bowl, combine flour, oats, brown sugar, salt, cinnamon, and cardamom. Work in cold butter until mixture resembles coarse crumbs. Stir in chopped pecans and refrigerate.

Toss sliced plums with lemon juice and zest, granulated sugar, and a pinch of salt. Let sit for 10 minutes, then mix in tapioca flour.

Spoon plum mixture into the prepared baking dish. Crumble the chilled topping over the plum filling.

Bake for 50-60 minutes until bubbling and golden brown. Check in the last few minutes.

Allow to cool for 30 minutes before serving.