Combine butter and white sugar in a bowl using your fingers; add flour, oats, 1 pinch cinnamon, and salt. Mix until crumbly.

Grease a microwave-safe mug. Place 1/2 oat crumble mixture in the mug; press to form a crust.

Microwave crust on high for 30 seconds.

Add apple slices, brown sugar, and 1 pinch cinnamon to the mug; add water and mix together carefully without disturbing bottom crust. Pour remaining 1/2 oat crumble mixture on top.

Microwave on 60 percent power for 1 minute 50 seconds. Cool before serving, about 1 minute.