Rinse the fish under cold water and pat very dry with paper towels. Using a sharp knife, trim any skin or connective tissue and cut the fish into uniform 1/2-inch cubes so they cook evenly in the citrus.

Make the marinade

In a nonreactive bowl (glass or stainless steel), combine the fresh lime juice, lemon juice (if using), kosher salt, and sugar. Stir until salt and sugar dissolve.

Cure the fish

Add the fish cubes to the citrus marinade, making sure the fish is fully submerged. Cover and refrigerate. For a quick ceviche with slightly translucent but tender fish, marinate 15–20 minutes. For firmer, fully 'cooked' pieces, marinate 1–2 hours. Do not exceed 3 hours to avoid an overly firm texture.

Aromatics and Vegetables

Prepare the aromatics

While the fish marinates, thinly slice the red onion and rinse briefly under cold water to soften bite and reduce sharpness; drain well. Seed and finely mince the jalapeño, remove seeds if you prefer milder heat. Seed and dice the tomato and cucumber. Chop the cilantro.

After the fish has marinated for the chosen time, drain off about half of the citrus marinade (reserve a little to keep the ceviche moist). Add the sliced onion, minced jalapeño, diced tomato, diced cucumber, and chopped cilantro to the bowl with the fish. Toss gently to combine, taking care not to mash the fish.

Finish and Serving

Fold in the extra-virgin olive oil and season with freshly ground black pepper and additional salt to taste (start with 1/4 tsp and adjust). If using avocado, fold in the diced avocado last to avoid browning and keep pieces intact.

Let the assembled ceviche rest in the refrigerator 5–10 minutes to let flavors marry, but serve cold. Spoon onto chilled plates or into small bowls.

Serve immediately with tortilla chips or tostadas. Garnish with a few extra cilantro leaves and lime wedges if desired.