Keeping crumble topping crisp is all about balancing moisture and heat.
Start by making sure the butter is cold when adding it to your crumble mixture, so it doesn’t spread as the crumble bakes.
There also needs to be a good balance of butter, flour and sugar in the topping mixture. Too much flour and you’ll end up dry, sandy patches that won’t crisp up. Too much butter, and the topping will melt.
Avoid covering your crumble topping with foil or a lid, which can trap steam and make the topping soggy.
Bake the crumble at the right temperature, typically between 350-375°F, and let the crumble cool on a wire rack after baking. This allows steam to escape, preventing the topping from becoming soggy.
Remember, crumbles are best enjoyed freshly baked when the topping is at its crispiest.