Hi Hotbot,
It depends. You need to achieve the correct consistency for the base (i.e like the consistency of soup). If the recipe is correct (i.e. if it defines the correct amount of liquid), then simmer with the lid on.
If the base appears too thin, simmer with the lid off.
If the base appears too thick, add more liquid.
.....but, don't panic! You have plenty of time......you really cannot simmer the base for too long......you want the oil to separate and rise to the surface......

Good Luck!
