In a 10-inch cast iron skillet, add berries, rhubarb, sugar, butter, flour, cornstarch, vanilla and salt. Cook on stovetop over medium heat, stirring regularly, until bubbly and thickened.
Meanwhile, in a stand mixer with paddle attachment, add the 1 cup flour, oats, sugar, cardamom and salt. Add the butter and mix until mixture holds together in coarse clumps. Adding a splash of water if needed.
When fruit is ready, drop large tablespoons of dough on top of warm fruit. Place into preheated oven and bake for 35-40 minutes, until the edges are bubbling and the top is golden brown.
Serve warm as is or topped with ice cream.
Leftovers: You do not need to refrigerate this cobbler recipe. Simply cover the cobbler and let it sit at room temperature. It is best consumed within 2 days.