In a medium bowl, stir together the brown sugar, old-fashioned oats, all-purpose flour, and salt. Drop in the cold butter cubes. Squish the butter between your fingertips, coating them with the oats mixture until crumbly.
Top the fruit filling with the crumble topping and bake.
Notes
Hint: Keep your crumble topping cold until it goes into the oven. If the butter softens too much, the topping can melt into the fruit instead of baking into those golden, crunchy crumbles we all love.Balance the Sweetness: Adjust the sugar depending on your fruit. Tart apples or berries love a little extra sweetness, while peaches or pears often need less. This keeps your crumble topping from overpowering the natural flavor of the fruit.