If using frozen shrimp, thaw completely under cold running water. If using raw shrimp, bring a pot of salted water to boil, add shrimp and cook 2-3 minutes until just pink. Drain and rinse with cold water to stop cooking.
Cut shrimp into bite-sized pieces (about 1/2-inch pieces) and place in a large glass or ceramic bowl.
Pour lime juice and lemon juice over shrimp, making sure all pieces are covered with citrus. Let sit for 10 minutes to begin the 'cooking' process.
Add diced red onion, cucumber, jalapeño, cilantro, salt, and pepper to the bowl. Stir gently to combine all ingredients.
Let ceviche marinate in refrigerator for at least 15 minutes, or up to 24 hours for best flavor development.
Just before serving, taste and adjust seasoning with additional salt, pepper, or lime juice as needed. Gently fold in diced avocado.
Serve chilled with tortilla chips, crackers, or lettuce cups. Garnish with extra cilantro and lime wedges if desired.
Notes
Storage tip: Keeps in fridge for up to 3 days in an airtight container. The shrimp will continue to 'cook' in the acid, becoming more opaque over time. Add avocado just before serving to prevent browning. For milder heat, remove jalapeño seeds completely or serve on the side.