Slice the red onion thinly with the grain and toss in a bowl with salt, pepper, and lime juice, coating well.
Add the fish, garlic, and fresh chilies, and gently mix.
Add the tomatoes, cucumber, cilantro, and olive oil, give a stir, and marinate in the refrigerator for at least 30 minutes before serving (45-60 minutes is ideal). The longer you marinate, the firmer and more “cooked” the fish will become.
Before serving, taste and adjust the salt and heat. Add more salt or chilies if you like. If adding avocado, gently fold it in right before serving—you may need to add a pinch more salt.
To make the optional Avocado Sauce, blend all ingredients in a blender until smooth, adding a bit more water or oil if needed to get the blades going.