The recipe below for the perfect crumb topping is slightly adapted from a pie crumb recipe on All Recipes to turn it into a crumb only recipe:
Using your hands, mix equal parts brown sugar and flour with half as much unsalted butter (e.g. 1/2 C brown sugar, 1/2 C flour, 1/4 C butter). It should stick together and form crumbs.
Spread the mixture onto a baking sheet lined with parchment paper.
Bake at 350 F for 20 minutes.
Crumble any pieces that stuck together and put the crumbs on everything in sight!