Squeeze enough fresh limes to measure 1 cup of juice. Add 1 tablespoon of fresh lemon juice. Remove any seeds. Pour the citrus juice over the cubed fish so the fish is fully submerged.

Marinate

Cover the bowl and refrigerate. Stir gently after 10 minutes to ensure even contact with the citrus. Continue marinating until the fish turns opaque and firms up — about 20–30 minutes total for 1/2-inch pieces. Taste a small piece at 20 minutes to check doneness; timing can vary with fish thickness.

Prepare vegetables and aromatics

While fish marinates, thinly slice the red onion into half-moons. Rinse the onion in cold water for 5 minutes if you prefer milder flavor, then drain well. Dice the red bell pepper and cucumber, mince the jalapeño (remove seeds to reduce heat), and chop the cilantro finely.

Combine and season

Once the fish is opaque, pour off about half of the citrus if you prefer a less acidic ceviche, leaving enough to coat and keep the fish moist (about 1/3 to 1/2 cup). Add the sliced red onion, diced bell pepper, diced cucumber (if using), minced jalapeño, and chopped cilantro to the bowl. Season with 2 tsp sea salt and 1 tsp freshly ground black pepper. Add 2 tsp extra-virgin olive oil if using. Gently fold to combine without breaking up fish.

Chill briefly

Cover and refrigerate the combined ceviche for another 10–15 minutes to let flavors marry. Adjust seasoning after chilling: add more salt, pepper, or a squeeze of lime if needed.

Add avocado

Just before serving, fold in the diced avocado gently so it does not brown or mash.

To Serve

Spoon the ceviche into shallow bowls or small glasses. Garnish with a few cilantro leaves and serve with tortilla chips or tostadas and lime wedges on the side. Enjoy immediately — ceviche is best the same day it’s made.