1. Preheat oven to 350 F.
2. Make filling. To a 9-inch square glass or ceramic baking pan, add apple, rhubarb, sugar, flour, orange juice and orange zest. Gently toss together. Use spatula to spread fruit evenly in baking pan.
3. Make crumble topping. In mixing bowl, add brown sugar, flour, rolled oats and salt. Use pastry blender or your fingers to cut butter into oat mixture until well combined and crumbly. Sprinkle oat mixture evenly over fruit.
4. Bake until crumble topping is golden brown, about 45 to 50 minutes.
5. Let cool for about an hour to allow juices to thicken.
6. Serve warm topped with vanilla ice cream, Greek yogurt or whipped cream. Makes 8 servings.