Add three whole cloves of garlic to a small container with just enough water to cover the garlic. Microwave for 1 minute (with the lid to the container on, loosely). Let cool.
Squeeze the garlic out of its peels into a molcajete, then add 2 tbsp. of diced onion, and all of the cilantro and peppers. Grind it all into a coarse paste.
Cut the avocados in half and remove the pits. Scoop the flesh out into the molcajete with a spoon. Add the salt, then mash with a fork, being sure to mix in the serrano paste. Do not mash it too smoothly. You want some texture.If your molcahete isn't large enough, You can put everything into a larger bowl before mashing. Try to get as much of the liquid out of the molcajete as possible.
Cut the tomatoes longways into quarters, then scoop the seeds out with a spoon and discard them. Finely dice the tomatoes. Stir the tomatoes, remaining white onion, and lime juice into your guacamole. Serve immediately.