Chop the serrano ham in cubes and cook it in the milk on medium heat for about 3min from the point where it starts steaming.
Cook the onion in a bit of the olive oil from the recipe over medium-low heat until soft and translucent.
Add the garlic and cook for a few seconds. Don't let it become golden.
Add the rest of the olive oil, salt, nutmeg and flour and mix using a whisk over medium heat.
Cook the mix for a minute from the moment it starts boiling.
Turn the heat off and add the milk little by little and whisking in between to make sure no lumps form.
Once all the milk has been incorporated return to a medium heat and whisk constantly until it starts boiling.
Add the manchego cheese and mix.
Transfer to a container and cover with cling film. Make sure the cling film touches the surface of the bechamel sauce so a skin doesn't form.
Once cooled to room temperature take to the fridge for at least 2 hours and a maximum of 4 days.