Peel and devein the shrimp if necessary, then rinse and drain them well. Place in a mixing bowl.
Using a citrus press or juicer, squeeze fresh lime juice from the limes until you have about half a cup. Pour the lime juice over the shrimp, ensuring they are fully submerged.
Thinly slice the red onion and add it to the bowl with the shrimp. Toss gently to coat everything evenly. Let this marinate for about 10-15 minutes until the shrimp turn opaque and firm, listening for that satisfying sizzle and watching for the color change.
While the shrimp is marinating, dice the ripe tomatoes and chop the cilantro leaves. Set aside.
Finely chop the jalapeño, including seeds if you want extra heat, and add it to the bowl with the other prepared ingredients.
Once the shrimp has turned opaque and firm, add the diced tomatoes, chopped cilantro, and jalapeño slices to the bowl. Gently fold everything together, ensuring the flavors meld and the ingredients are evenly distributed.
Season with salt and pepper to taste, tasting and adjusting to achieve a bright, balanced flavor. Let sit for a few minutes to allow flavors to develop further.
Serve the ceviche chilled, garnished with extra cilantro or lime wedges if desired. Enjoy the fresh, zesty, and firm-textured dish immediately for the best experience.