Cook the Shrimp (Optional):
If you prefer cooked shrimp, blanch the shrimp in boiling water for 1–2 minutes until pink, then cool in an ice bath. If using raw shrimp, ensure they are very fresh.
Marinate the Shrimp:
In a large bowl, combine the shrimp with the lime and lemon juices. Cover and refrigerate for 30–60 minutes, stirring occasionally, until the shrimp turns opaque (if using raw shrimp).
Prepare the Vegetables:
While the shrimp marinates, dice the onion, tomato, cucumber, avocado, and pepper (if using). Chop the cilantro.
Combine Ingredients:
Once the shrimp is ready, add the onion, tomato, cucumber, jalapeño, and cilantro to the bowl. Toss to combine.
Season:
Season the ceviche with salt and pepper to taste. Gently fold in the avocado just before serving to prevent it from becoming mushy.
Serve:
Serve chilled with tortilla chips, on tostadas, or in small glasses as an appetizer.