In a cup, mix the apple and the brown sugar.
In a small bowl, place the oatmeal, brown sugar and flour. Add the butter and mix with your fingertips until the texture is lumpy. Put this mixture on the apple. Cook in the microwave for 1 to 1 1/2 minutes or until the juice is bubbling
Be careful, it’s hot! Let it cool for at least 5 minutes before serving.