If using fish: rinse under cold water and pat dry. Trim skin and any dark bloodline then cut into uniform 1/2-inch cubes. If using raw shrimp: peel, devein, rinse, pat dry and cut into 1/2-inch pieces (or leave small shrimp whole). Place seafood in a nonreactive glass or ceramic bowl.

Marinade

Pour the fresh lime juice and lemon juice over the seafood until fully submerged or well-coated (about 3/4 to 1 cup juice per pound). Sprinkle kosher salt and sugar over the mixture and gently stir to combine. Ensure all pieces are in contact with the citrus.

Cover the bowl with plastic wrap and refrigerate. For shrimp: marinate 15–20 minutes until opaque and firm. For fish: marinate 20–30 minutes, depending on cube size, until fish turns opaque and flakes slightly when pressed. Do not exceed 2 hours to avoid overcooking from the acid.

Vegetables & Mixing

While the seafood marinates, thinly slice the red onion and place it in a small bowl of cold water for 5–10 minutes to mellow sharpness, then drain and pat dry.

Dice the tomato and cucumber, mince the jalapeño (remove seeds for less heat), and chop the cilantro. Combine the drained onion, tomato, cucumber, jalapeño, and cilantro in a mixing bowl.

When the seafood has finished marinating, pour it through a fine mesh sieve to remove about half of the citrus (reserve a few tablespoons of the citrus if you want a saucier ceviche). Transfer the seafood to the bowl with vegetables and gently fold to combine.

Season & Finish

Taste and adjust seasoning: add ground black pepper and additional salt if needed. If desired, drizzle the teaspoon of extra-virgin olive oil and gently fold.

Gently fold in the diced avocado just before serving to avoid browning. Garnish with lime wedges and a few cilantro leaves.

Serving

Serve immediately chilled in bowls or on tostadas or gluten-free crackers. Leftovers: store in an airtight container in the refrigerator and consume within 24 hours; the texture will continue to change as acid 'cooks' the seafood.