Pat the fish fillet dry with paper towels. Trim any skin or connective tissue and cut the fish into 1/2-inch to 3/4-inch cubes so the acid will penetrate evenly.

Marinate in citrus

Place the fish cubes in a nonreactive bowl (glass or stainless steel). Pour the lime and lemon juice over the fish until mostly submerged. Stir gently to coat. Cover and refrigerate for 20–30 minutes, checking for doneness: the exterior should look opaque and slightly firm. (If you prefer a firmer 'cooked' texture, marinate up to 45 minutes; avoid over-marinating or the fish will become tough.)

Prepare vegetables and seasoning

While the fish is marinating, rinse and thinly slice the red onion and soak in cold water for 5 minutes to reduce sharpness, then drain. Halve the cherry tomatoes. Finely mince the serrano/jalapeño (remove seeds for less heat). Chop the cilantro. Dice avocado just before serving to prevent browning.

Combine and season

After the fish is opaque, drain about half of the citrus (leave a few tablespoons for flavor). Add the sliced onion, cherry tomatoes, minced pepper, cilantro, salt, and black pepper to the bowl with the fish. Add the olive oil if using. Gently fold to combine. Taste and adjust salt, pepper, or more lime juice if needed.

Rest and serve

Let the assembled ceviche sit refrigerated 5–10 minutes for flavors to meld. Just before serving, fold in diced avocado. Serve chilled with tortilla or plantain chips and lime wedges; garnish with extra cilantro.

Ceviche de Thon (Tuna Ceviche)

Prepare the tuna

Use very fresh, sushi-grade tuna. Pat dry and cut into uniform 1/2-inch cubes. Place in a chilled bowl.

Make the citrus-soy dressing

In a small bowl, whisk together lime juice, orange juice, soy sauce, and sesame oil until combined. Taste and adjust balance — more lime for brightness, more soy for saltiness.

Combine tuna and aromatics

Add the diced cucumber, sliced green onion, and minced red chili to the tuna. Pour the dressing over the tuna and gently toss to coat. Sprinkle the toasted sesame seeds, salt, and black pepper; toss lightly. Because tuna is delicate and the acid level is milder here (shorter 'cook'), marinate only 5–10 minutes in the refrigerator so the cubes remain tender and slightly raw inside.

Plate and garnish

Spoon the tuna ceviche into chilled bowls or onto small plates. Garnish with microgreens or thin radish slices and a few extra sesame seeds. Serve immediately with lime wedges and reserved chips or crispbread.

Common finishing steps

Taste both ceviches before serving and adjust seasoning with sea salt, extra lime juice, or a drizzle of olive oil as desired.

Keep leftovers refrigerated and consume within 24 hours for best quality. Note: ceviche relies on acid, not heat, to change texture; always use the freshest fish and sushi-grade tuna when serving raw or lightly 'cured' fish.