Slice the shrimp lengthwise to create two thin pieces, then chop each slice into 3-4 smaller chunks, depending on your desired chunkiness.

In a food processor, combine the lime juice, lemon juice, and one serrano pepper (with seeds removed). Blend until the mixture is smooth.

Combine the shrimp in a large bowl with the serrano pepper-citrus blend, red onion, cilantro, diced serrano pepper (without seeds), cucumber, oregano, salt, and pepper. Carefully mix the ceviche ingredients. Cover and chill for at least an hour, allowing the flavors to meld and the shrimp to be "cooked" by the juice's acidity.

Prior to serving, incorporate the avocado and season with additional salt and pepper to taste.