Add plenty of citruses, like lemon or lime. This will help to slow down the oxidation process. Reserve the large seeds and add them to the guacamole if you must store it in the fridge before serving. Then, when ready, remove the seeds and feed your guests. Put a thick circle slice of onion on top of the guac. Simple enough. Onions contain sulfur—which is an element that helps to slow down the oxidation process in avocados.