Add flour, brown sugar and oats to a medium bowl and stir to combine.
Melt ½ cup butter.
Pour butter over flour, brown sugar and oats and stir with a fork until thoroughly combined.
Pour the crisp topping over apples and bake!
Notes
Sheet Pan Crisp Topping: Spread the crumble evenly on a parchment-lined sheet pan and bake at 350°F (177°C), stirring once or twice, until golden and crisp. Sprinkle it over yogurt, ice cream or roasted fruit for a sweet, crunchy finish.