Combine the onion with the juice of 2 limes in a small bowl; allow the onion to soak in the lime juice for 2 hours. Strain through a fine-mesh sieve and discard the lime juice. Set the onions aside.
Grind the serrano chile peppers, the juice of 2 limes, and salt together in a food processor until smooth; add the avocado and continue processing until creamy and smooth.
Transfer the mixture into a small serving bowl; fold the soaked onions, chopped cilantro, and about half the pomegranate seeds into the avocado mixture. Top with the remaining pomegranate seeds and garnish with the cilantro sprig to serve.
