Toss together the shrimp, red onion, and jalapeno with lime juice. Place in a shallow glass dish or bowl where the shrimp can soak in the lime juice. Make sure the shrimp is covered in lime juice. Let sit for at least 20-30 minutes, but no longer than 3 hours. If you want to use raw shrimp, soak the shrimp and red onion only in the lime juice for 30-40 minutes, flipping once, until the shrimp is opaque and pink. Then stir in the remaining ingredients.
When ready to serve, stir in the cucumbers, tomatoes, avocado, and cilantro., Season well with salt and pepper. If the ceviche feels too sour, add a drizzle of olive oil. This is best eaten immediately as the ingredients break down as it sits.