If you find the ceviche a little too acidic, drain out some of the juices after the marinating, add a little more avocado (or some olive oil) and/or a little more salt.
1 pound medium-small shrimp, peeled and deveined
2 Tbsp salt
3/4 cup lime juice (juice from 4-6 limes)
3/4 cup lemon juice (juice from 2-3 lemons)
1 cup finely chopped red onion
1 serrano chile, ribs and seeds removed, minced
1 cup chopped cilantro
1 cucumber, peeled diced into 1/2-inch pieces
1 avocado, peeled, seed removed, cut into 1/2-inch chunks
1 In a large pot, bring to a boil 4 quarts of water, salted with 2 Tbsp salt. Add the shrimp and cook for 1 minute to 2 minutes max, depending on size of shrimp. (Over-cooking the shrimp will turn it rubbery.) Remove shrimp with a slotted spoon and place into a bowl of ice water to stop the cooking.
2 Drain the shrimp. Cut each piece of shrimp in half, or into inch-long pieces. Place shrimp in a glass or ceramic bowl. Mix in the lime and lemon juice. Cover and refrigerate for a half hour.
3 Mix in the chopped red onion and serrano chile. Refrigerate an additional half hour.
4 Right before serving, add the cilantro, cucumber, and avocado.