Using a Mexian molcajete or mortar and pestle, smash together your onion, serrano, garlic and half of your cilantro with some coarse salt into a rough paste. Note: This step is optional but does greatly help the overall flavor distribution in your Guacamole. But, in a pinch, it will still be very tasty without this step.
In a large mixing bowl, mix together your avocados, aromatic paste, the remaining cilantro, cumin and lime juice. Season to taste with salt and more lime juice if needed. Note: You can use a large serving fork to mash everything up.
Serve in a bowl garnished with cilantro leaves, a squirt of lime juice and a sprinkle of coarse salt.
Use immediately or cover snugly in plastic wrap, pressing it into the Guacamole to make sure there is no air stuck between them so that it does not brown. If storing overnight, also put in an airtight container. More lime juice will also help it keep over a few days, see notes above.
