For the filling first stew the rhubarb till just soft in a little water (about half a cup) - I usually do this in the microwave to save time as I'm making the crumble topping. For sweet tasty rhubarb I add about 1/2 cup of raw sugar and a couple of orange quarters for flavour in the cooking then remove.
Combine pie apple and stewed rhubarb, add cinnamon and nutmeg if liked and place in a large ovenproof casserole dish.
Combine topping ingredients by hand adding the melted butter, any extra sugar and spices if liked.
Sprinkle over apple/rhubarb mixture finishing with flaked almonds.
Place in preheated moderate oven for about 20 to 30 mins or until nicely golden and crunchy on top.
Can omit rhubarb for an even quicker dessert and just add a handful of sultanas to the apple and increase the cinnamon if liked.