Chop the onions and add them to a small sauce pan.
Add the butter.
Gently cook the onions on the stovetop in the butter on very low heat until they are translucent. Stir often so that the onions do not brown.
Season the onions with the salt, pepper and nutmeg.
Add the cream to the pan. Stir until the cream is warm.
Put the contents of the pan into a blender. Blend until smooth.
Return the contents of the blender to the pan. Cook on low heat for about 5 minutes so that the sauce can reduce and firm up a little.
Set aside and allow the sauce to cool down for a few minutes before topping the pizza.