To make 2 cups of simple crumb topping for pie, mix 1 cup brown sugar and 1 cup all-purpose flour in a medium sized bowl. Using a fork or stand mixer, slowly add 1/4 cup of butter just until the topping is crumbly. Use the mixture to top a pie just before baking. Adjust the recipe size based on the size of the pie.
To make an alternative crumb topping, combine 1 cup all-purpose flour and 1/2 cup brown sugar in a medium bowl. Add 1/2 cup of cold butter with a pastry blender until the mixture turns into coarse crumbs. Optionally, stir in 2/3 cup of chopped walnuts and 1/2 cup of dried apples. Depending on the pie, you can also add a small amount of cinnamon or nutmeg to taste. Use the crumble to top a pie just before baking.
Another more in-depth option is to make a crumb topping with oatmeal. Any type of oatmeal works, except instant. To make 2 cups, combine 2/3 cup brown sugar, 3/4 cup flour, 3/4 cup oatmeal, 1/2 teaspoon cinnamon and 1/4 teaspoon salt in a large mixing bowl. Add 1/3 cup of cold butter with a pastry blender until the mixture turns to coarse crumbs. Use the crumble on top of any pie, add just before baking.