Pile the onion, serranos, cilantro and a big pinch of kosher salt on a cutting board. Chop, then mash into a paste with the flat side of a large knife. Transfer to a medium bowl, add the avocados and mash with a wooden spoon until combined but still chunky. Stir in the tomato and season with salt. Garnish with more tomato, cilantro and onion.