Preheat the oven to 360°F. Lightly grease a medium baking dish (around 8x8 inches or similar).
Prepare the fruit filling:
Add the sliced apples, chopped rhubarb, and granulated sugar to the prepared baking dish. Toss to combine and spread into an even layer.
Make the crumble topping:
In a medium bowl, combine oats, flour, brown sugar, and salt. Add the cold plant-based butter and use your fingertips to rub it into the dry ingredients until the mixture resembles coarse crumbs.
Assemble & Bake:
Sprinkle the crumble topping evenly over the fruit mixture.
Bake for 40–45 minutes, or until the top is golden brown and the edges are bubbling.
Let the crumble sit for 5–10 minutes before serving. Serve warm with vegan ice cream or custard if desired.