01 - Preheat oven to 350°F (175°C) and grease a 12-cup muffin pan.
02 - In a medium bowl, combine 1 cup oats, 1/2 cup flour, 1/3 cup brown sugar, 1/2 teaspoon cinnamon, and 1/4 teaspoon salt. Stir in melted butter until evenly moistened.
03 - Divide the oat mixture evenly among muffin cups. Firmly press mixture into the base and halfway up the sides to create a well.
04 - In a separate bowl, toss diced apples with lemon juice until coated. Spoon a generous amount into the center of each oat cup.
05 - In another bowl, combine 1/2 cup oats, 1/4 cup flour, 1/4 cup brown sugar, and 1/2 teaspoon cinnamon. Add cold butter cubes and mix until crumbs are formed.
06 - Evenly distribute crumble mixture over apples. Bake for 22-25 minutes, or until topping is golden and apples are tender.
07 - Allow crumble cups to cool in pan for 10 minutes, then carefully remove and serve warm or at room temperature.