Preheat your oven to 350°F (175°C) and butter a 9-10 inch skillet or baking dish.

Combine all-purpose flour, thick rolled oats, brown sugar, fine sea salt, cinnamon, and cardamom for the topping. Work in the cold butter until crumbly.

Toss the plums with lemon juice and zest, granulated sugar, and salt. Let sit for 10 minutes and stir in tapioca flour.

Spoon the plum filling into the prepared dish and crumble the topping over evenly.

Bake for 50-60 minutes until the fruit bubbles and the topping is golden.

Cool for about 30 minutes before serving warm with optionally ice cream or whipped cream.