Peel the sweet potato, slice into 1 cm and soak in water. Peel the apple and slice thinly. Put both in a pot and add enough water to cover plus sugar, and a few drops of lemon juice if you have it. Heat on medium (see the Helpful Hints).
Once it's cooked enough so that the sweet potato falls apart and the moisture has reduced by 1/3, stop the heat, add butter, and mix.
Transfer to a plate and it's complete! Eat as-is, or sprinkle on cinnamon. It's also delicious while cold.