Boil rocoto pepper 1 minute; drain. Repeat twice. Combine 1 1/4 ounces pepper flesh (reserve remaining pepper for another use) and 1 teaspoon pepper seeds, 1 1/4 ounces fish, 1/4 cup chopped red onion, juice, salt, garlic, celery, and ginger in a blender. Process until smooth. Chill rocoto mixture for 20 minutes.
Combine chilled rocoto mixture, remaining 24 ounces sea bass, sliced red onion, and cilantro in a bowl; toss to combine. Serve in chilled bowls or glasses.