Slice the red onion very thinly and salt generously and let stand 15 minutes until it begins to release its liquid (this will remove the bitterness). Rinse well, squeeze dry.
Place fish, garlic, onion, salt, pepper, fresh chilies, and lime juice in a shallow serving bowl, gently mix, and marinate in the refrigerator for at least 30 minutes before serving. The longer you marinate the firmer and more “cooked” the fish will become.
Before serving, gently toss in the fresh cilantro, cucumber and tomato and a drizzle of olive oil, gently mix.
Taste for salt and add more if necessary. If adding avocado, gently fold it in at the end, after everything is mixed, making sure to use one that is not too soft.
To Serve: Serve ceviche in a big bowl with chips on the side, or as mini tostadas (on a round chip or homemade) or make ceviche tacos or ceviche lettuce cups.
Serve ceviche in small appetizer cups or shot glasses with a mini fork.
To make crispy mini tostadas: lightlyspray mini corn tortillas ( 3 inch tortillas) with olive oil and season generoulsy with salt. Bake in a 300 F oven on a sheet pan until crisp (not chewy, but crisp like a cracker) about 25 -30 minutes, turning halfway through. Let cool completely.