Note
For filling use any fresh fruit of your choice - like - peaches, pears, strawberries, plums.
You can even make it from a combination of the fruits. The above pic is a combination of apples and plums.
Instead of pre cooking the filling, you can mix all the filling ingredients in a bowl and set aside for 10 to 30 mins for flavors to deepen. Then put into greased baking dish, cover with crumble and bake.
You can substitute flour and oats with about 1 cup wheat flour or mix of oats and wheat flour. But you may need to increase the butter quantity to get the crumbly mixture - up to 80 to 100 gms.