Pile 1/4 cup chopped onion, 1 to 3 chopped serrano chiles, 3/4 cup torn 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 bowl, add 3 diced avocados and mash with a wooden spoon until combined but still chunky. Stir in 1 medium diced tomato and season with salt. Top with more tomato, cilantro and onion.