Step 1: Preheat your oven to 375°F (190°C). Lightly grease an 8×8 inch baking dish or similar-sized oven-safe dish.
Step 2: In a large bowl, combine the chopped rhubarb and apples. Add 2 tablespoons of maple syrup, 1 tablespoon of cornstarch (or arrowroot powder), and 1 teaspoon of vanilla extract. Toss gently until the fruit is evenly coated. Pour the fruit mixture into the prepared baking dish, spreading it evenly.
Step 3: In a separate medium bowl, combine 1 cup of rolled oats, 1/2 cup of whole wheat flour, 1/4 cup of chopped walnuts or pecans (if using), 1/4 cup of brown sugar (or coconut sugar), 1 teaspoon of ground cinnamon, and a pinch of salt. Mix well.
Step 4: Add 1/4 cup of cold, unsalted butter (or coconut oil for a vegan option), cut into small cubes, to the dry ingredients. Use your fingertips or a pastry blender to cut the butter into the dry ingredients until the mixture resembles coarse crumbs. Some larger clumps are desirable.
Step 5: Generously sprinkle the crumble topping over the fruit, ensuring it covers the entire surface.
Step 6: Place the baking dish on a baking sheet (to catch any potential drips). Bake for 35-45 minutes, or until the fruit is tender and bubbling, and the crumble topping is golden brown and crispy. If the topping starts to brown too quickly, you can loosely cover it with foil.
Step 7: Allow the crumble to cool for at least 15-20 minutes before serving. This allows the filling to set properly. Serve warm, perhaps with a dollop of Greek yogurt or a scoop of healthy vanilla ice cream.