Combine butter and white sugar in a bowl using your fingers. Add flour, oats, 3 dashes cinnamon, and salt; mix until crumbly.
Grease a mug. Flatten half the crumble mixture into the bottom to form a crust.
Microwave crust for 30 seconds on high.
Remove mug from microwave; add 1 dash cinnamon, apple slices, and brown sugar. Add water. Mix apples, brown sugar, and cinnamon together carefully without disturbing crust. Pour remaining crumble mixture on top.
Microwave for 1 minute and 50 seconds on 60 percent power. Remove from microwave; let cool about 1 minute.
