PREPARE THE CRISP TOPPING

1. Preheat the oven to 350°F.
2. In a medium mixing bowl, combine the rolled oats, gluten-free 1:1 flour, light brown sugar, cinnamon, and salt. Stir together until well mixed.
3. Add the cold, cubed butter and toss to coat. Using your fingers, work the butter into the dry ingredients until the mixture is crumbly and resembles wet sand. Small, pea-sized pieces of butter are fine, but there should be no large chunks remaining.
4. Stir in the chopped walnuts. Set aside.

PREPARE THE FILLING

1. Trim the rhubarb and slice into 1-inch pieces.
2. Core the apples and cut them into 3/4- to 1-inch chunks.
3. In a large mixing bowl, combine the apples, rhubarb, granulated sugar, light brown sugar, vanilla, cornstarch, cinnamon, cardamom, and ginger. Toss until the fruit is evenly coated.

ASSEMBLE AND BAKE

1. Transfer the fruit mixture to a 9-inch square or round ceramic baking dish. (An 8-inch dish will also work—fill it as full as possible; you may have a small amount of leftover filling.)
2. Sprinkle the crisp topping evenly over the fruit.
3. Bake on the middle rack for 40–50 minutes. Check at 30 minutes and loosely tent with foil if the topping is browning too quickly. The crisp is done when the topping is golden brown and the fruit juices are bubbling around the edges.
4. Remove from the oven and place on a wire rack to cool for about 30 minutes before serving.
5. Serve warm with a scoop of vanilla ice cream.

Store the crisp covered in the refrigerator for up to 4 days. Reheat in a 350°F oven until warmed through before serving.