Combine in a food processor achiote paste, olive oil, cumin, coriander, oregano, and the 1-tablespoon of garlic. Make sure to puree all these ingredients until it becomes a paste. Rub the marinade inside the chicken cavities, underneath the skin, as well as, outside. Allow the chickens to marinade overnight, refrigerated.
Next day, heat up a large saute pan. Add chorizo and cook for 5 minutes, until crispy, then add the onion and 4 tablespoons of garlic, and cook for another 2 minutes. Add mustard greens and deglaze with chicken stock. Cook for 2 minutes until greens have wilted, then add potato, green olives, and season to taste with salt and pepper. Once the stuffing has cooled, distribute it equally into the chickens and place them on a sheet with a rack. Place in a preheated 375 degree F oven for 1 hour.