If using fish: rinse and pat dry. Trim away skin and any bloodline, then cut into 1/2-inch dice. If using shrimp: peel, devein and rinse; cut larger shrimp into 1/2-inch pieces. Place seafood in a glass or ceramic bowl (never aluminum).

Add citrus to cure

Pour the lime juice and lemon juice (if using) over the diced seafood until fully submerged. Stir gently to ensure all pieces are coated. Cover and refrigerate for 20–30 minutes for shrimp or 30–45 minutes for fish, stirring once halfway. The seafood should appear opaque and slightly firm when 'cooked' by the acid.

Prepare vegetables

While seafood cures, thinly slice the red onion and rinse briefly under cold water if you want to mellow its bite; drain well. Seed and finely chop the jalapeño/serrano (leave seeds for more heat). Dice the tomato, peel/seed/dice the cucumber if using, and chop the cilantro. Put the prepared vegetables in a bowl and season with kosher salt and black pepper.

Combine and season

Once the seafood is opaque and firm, pour it through a fine-mesh strainer, reserving about 2–4 tablespoons of the citrus 'marinade' (adjust to taste). Return the seafood to the bowl and add the reserved marinade, the vegetables (onion, chili, tomato, cucumber), chopped cilantro, olive oil, and hot sauce (if using). Gently fold to combine. Taste and adjust seasoning with additional salt, pepper, or lime juice as needed.

Chill briefly

Cover and refrigerate the combined ceviche for 10–15 minutes to let flavors meld (do not marinate much longer or the seafood can become overly firm).

Add avocado and serve

Just before serving, gently fold in the diced avocado. Serve cold with tortilla chips, on tostadas, or in lettuce leaves. Garnish with extra cilantro and lime wedges if desired.

Storage note: Leftover ceviche is best eaten the same day; store in an airtight container in the refrigerator for up to 24 hours, but texture will degrade as the acid continues to cure the seafood.