Step 1: Rinse the fresh fish under cold water and pat dry with paper towels before cutting it into uniform 1/2-inch cubes for your civeche recipe.
Step 2: Place the cubed fish in a non-reactive glass bowl and pour the fresh lime and lemon juice over it until the fish is completely submerged.
Step 3: Cover the bowl and refrigerate for 15 to 25 minutes, or until the fish becomes opaque and firm, which is the hallmark of a properly cured civeche recipe.
Step 4: While the fish cures, soak the sliced red onions in ice water for 10 minutes to remove their harsh bite, then drain and pat dry.
Step 5: Gently fold in the red onions, chopped cilantro, minced jalapeño, diced cucumber, and olive oil into the fish mixture.
Step 6: Season your civeche recipe with sea salt and black pepper to taste, then serve immediately with your choice of sides.