Preheat oven to 350 degrees F (175 degrees C). Line a baking sheet with aluminum foil.
Mix green bell pepper, red bell pepper, cilantro, olive oil, sazon seasoning, onion powder, adobo seasoning, salt, and black pepper together in a bowl. Place 1 clove garlic and 1 onion into the cavity of each hen. Spread seasoning-oil mixture with your hands over the entire outer surface of each hen. Arrange hens on prepared baking sheet; top each with 1 tablespoon butter.
Bake in the preheated oven until no longer pink at the bone and the juices run clear, about 45 minutes. An instant-read thermometer inserted into the thickest part of the thigh, near the bone should read 165 degrees F (74 degrees C).