In a pot with just enough water to cover the bottom, steam the mango slices until they are very soft. Keep a close watch to make sure water doesn't simmer away, leaving mangos to burn or stick. Puree the cooked mangos in a blender or food processor along with the sugar, or mash mangos and sugar together until all lumps are gone. Serve immediately, or chill.
Variation: Whip 1 cup of heavy cream and fold it into chilled mango puree before serving.