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 into the mixture and gently mix together.
Incorporate the tomatoes, cucumber, cilantro, and olive oil into the bowl. Stir gently, then marinate in the refrigerator for at least 30 minutes (45-60 minutes is ideal).
Before serving, taste and adjust salt and heat levels. Add more salt or chilies if needed. If adding avocado, fold it in right before serving and adjust salt as necessary.
Blend all avocado sauce ingredients in a blender until smooth. Add water or oil as needed for blending consistency.
Serve ceviche in a bowl with tortilla chips, as mini tostadas, tacos, lettuce cups, appetizer cups, or shot glasses with mini forks. Plate it with Avocado Sauce as a salad course.
Spray mini corn tortillas (3-inch tortillas) lightly with olive oil and season with salt. Bake in a 300°F (150°C) oven on a sheet pan until crisp (25–30 minutes), turning halfway through. Let cool completely.