Start by peeling and deveining the shrimp, then give them a quick rinse under cold water. Pat dry and set aside.
Juice the limes using a citrus press or your hands, collecting about 3/4 cup of fresh lime juice. Set aside.
Slice the red onion very thinly and soak the slices in cold water for 10 minutes to mellow their sharpness. Drain and set aside.
Chop the cilantro leaves finely and dice the jalapeño, removing seeds for less heat. Keep both ready for mixing.
Place the peeled shrimp in a glass or ceramic bowl and pour the fresh lime juice over them, ensuring they are fully submerged. Cover and refrigerate for 15-20 minutes, or until the shrimp turn opaque and firm.
Once the shrimp have turned opaque and firm, add the sliced onion, chopped cilantro, and minced jalapeño to the bowl. Gently fold everything together with a spoon.
Drizzle in the olive oil and sprinkle with a pinch of sea salt. Mix again to evenly distribute the flavors and shine.
Taste the ceviche and adjust with more salt or lime juice if needed. Serve immediately or chill for up to 4 hours for the best flavor and texture.