In a small bowl, thoroughly mash 1 tbsp chopped onion, the Serrano chile, 1/2 tsp cilantro and the salt with the back of a wooden spoon until it becomes a juicy paste.
Cut into the flesh of each avocado half, making slits both down and across to form a grid. Scoop out the avocado from the skin and place it in the bowl with the onion paste. Stir thoroughly to coat with the paste. Add the remaining onion and cilantro, and the tomato. Gently fold to incorporate all the ingredients. Add more chopped chiles and salt to taste.