For an extra layer of flavor, consider adding a pinch of red pepper flakes or a dash of your favorite hot sauce. If you prefer a less pronounced onion flavor, soak the chopped onion in cold water for 10 minutes, then drain well before adding to the ceviche. This ceviche is best enjoyed the day it's made, as the avocado may brown slightly over time. If making ahead, add the avocado just before serving. Experiment with other toppings! Diced tomatoes, cucumbers, or even a sprinkle of cotija cheese would be delicious additions.