This is the version of guacamole you often see in little taquerias in Mexico. The spicy, thin mixture has a great smooth flavor. But the reason you see it used so often is that it is much less expensive than the famous chunky guacamole, an advantage in tiny restaurants where tacos commonly cost less than the equivalent of thirty cents.

It is so good, however, that I use it even when the difference in cost is no object.

Put all the ingredients except the onion in a food processor and blend until very smooth. Pour into a bowl and stir in the onion. Use immediately.