Flour and oats are the base of crumble topping and it should be in 1:1 ratio.
Brown sugar is a better sweetener than white sugar due to its molasses content which brings a deep flavor.
Spices like cinnamon, nutmeg and cardamom brings a warming flavor to the crumble.
Cold butter makes the perfect crumbly, clumped textured for achieving the iconic consistency.
Bake the crumble before filling it with cream mixture for an extra crispy texture.