Defrost and drain shrimp. Transfer to a bowl. Add lime, and lemon juices. Stir in cucumber, red onion and Serrano chile. Refrigerate for an hour. Stir in tomato, avocado and chopped cilantro. Mix gently, taste and season with salt. Allow to sit at room temperature for about 20 minutes before serving.
Spoon ceviche into martini glasses or small bowls. Garnish with sprigs of cilantro and slices of lime. Serve with tortilla chips or fried plantain strips.