Oven to 350˚ F.
In a small bowl the apple with 1 tablespoon of sugar and cinnamon. Divide apple into 2 individual ramekins and press down flat.
Mix flour and 2 tablespoons sugar, then add melted butter in. Mix with a fork until mixture clumps together.
In oats and walnuts, if using. Sprinkle crumble evenly on top of the apple mixture.
Bake for 25-30 minutes, until topping is golden brown, and apples are tender. Best served immediately.
