In large baking dish, spread out the rhubarb and sliced apple, and sprinkle with the quarter-cup of coconut sugar.
In a large bowl, add the wholemeal flour, oats, butter, coconut, pinch of salt, and brown sugar. Rub the butter into the ingredients with your fingertips until well blended and the mixture is in small lumps.
Top the fruit with this mixture and place in the heated oven.
Bake 30-40 minutes until the crumble top is golden and the fruit below is cooked through. The juices should be bubbling along the edges of the dish and it should smell amazing!