Optional is chopped almonds, flax seeds, sunflower seeds.
To be honest I don't usually use a recipe I just throw in what looks good to me on the day.
If you don't like an ingredient leave it out or substitute another. Add more or less of the ingredients, it's a very forgiving recipe. I tend to throw in all of the above.
Toast the crumble on a tray by itself and sprinkle over any stewed fruit you like.
The crumble will freeze (before cooking, nuts and all). If I make to much I put it in a click clack container in the freezer, next time I'll either add more ingredients to it, or make another batch and throw the 2 leftovers together to freeze for another time.
Make a double batch of crumble, use 1 serve and freeze the other for a quick dessert any time. Label it.
I cook my apples to just softened with 1/2 teaspoon sugar. I also use half peaches from a tin.
Serve warm or cold, with ice cream or custard.