In a large mixing bowl, whisk together the brown sugar, flour, cinnamon, salt, and a pinch of cardamom until well combined. Next, using a pastry cutter or your fingertips, blend in the butter until the mixture forms a crumbly texture, similar to pea-sized clumps. Once you’ve reached the right consistency, fold in the rolled oats with a wooden spoon, ensuring everything is evenly mixed. The topping is now ready to be sprinkled over your apple filling, bringing a perfect crunch to your apple crisp.