Dry ingredients: Add the brown rice flour, oat flour, cornstarch, and brown sugar to a mixing bowl. Whisk to combine.
Make the crumble: Cut the butter into small cubes and add half of it to the mixing bowl. Use your fingers to pinch the butter and the dry ingredients together. Gradually add the remaining butter cubes, continuing to mix until the crumble batter has a dark brown color and crumbly texture (as shown in the picture).
Apples: Peel and core the apples. Cut them into small pieces and add them to a baking dish. Sprinkle with cinnamon and toss until evenly coated.
Bake: Crumble the mixture onto the apples. Bake for 40-45 minutes at 350°F (180°C, fan) until the top is golden brown.
Let it cool for 10 minutes to help it crisp up if you want to serve it warm or for 2-3 hours if you prefer it cold.
Notes
Vegan butter: Make sure to use plant-based block butter and not a vegan spread. I'm based in Europe and have had great results with Solo Gourmet (based on palm and sunflower oil) or Violife's unsalted butter. If you're successfully making this recipe with another vegan butter brand, please share it in the comments with us.
Brown sugar: If you're based in Europe, dark brown sugar does not refer to unrefined cane sugar. It refers to a slightly wet, dark brown sugar with a higher molasses content. It's often called "cassonade" in Belgium and Canada, and "vergeoise" in France. You can easily make your own by mixing 1 cup of crystallized white sugar (200 g) with 2 tablespoons of molasses. Combine with your hands or a spatula until there are no lumps and the sugar has the consistency of wet sand.