Start by placing the raw shrimp in a clean bowl and pour over the freshly squeezed lime juice, ensuring they are fully submerged. Set aside in the fridge for about 15-20 minutes until the shrimp turn opaque and firm, signaling they are lightly cooked by the citrus.
While the shrimp are marinating, prepare the aromatics. Use a sharp knife to thinly slice the red onion and finely dice the chili pepper. Chop the fresh cilantro and, if using, dice the ripe tomatoes.
Once the shrimp have turned opaque and slightly firm, transfer them along with the lime marinade into a mixing bowl. Add the sliced onions, diced chilies, chopped cilantro, and optional tomatoes.
Gently fold all the ingredients together, ensuring the shrimp are well coated with the citrus and herbs. The mixture should look vibrant and colorful, with the shrimp evenly distributed among the aromatics.
Cover the bowl with plastic wrap and refrigerate for at least 30 minutes to allow flavors to meld and the citrus to fully cook the shrimp. The ceviche will have a bright, tangy aroma and a slightly firm texture.
Give the ceviche a gentle stir before serving. Check that the shrimp is firm and opaque, and the flavors are bright and well-balanced. Serve chilled in individual bowls or on a platter, garnished with additional cilantro if desired.