For an extra layer of flavor, toast the oats in a dry skillet over medium heat for 5-7 minutes before adding them to the crumble topping. Feel free to experiment with different berry combinations, such as blueberries, raspberries, blackberries, and strawberries. If you don't have lemon zest, you can substitute it with a pinch of ground nutmeg or cardamom for a warm, aromatic note.