Cook the chicken in a litre of water with salt. Once the water boils, add quartered onions, carrots, potatoes and chayote. Keep the pot covered and boil until cooked. Remove the chicken and the vegetables and the set the stock aside.
In a separate frying pan, roast the black peppercorns, cinnamon, allspice berries, cloves and coriander seeds over medium heat until fragrant. Transfer these into a bowl when roasted and then roast the pepitoria, chillies, sesame seeds and oregano in a similar way.
Grind all the roasted ingredients and spices into a powder.
Heat the oil in the frying pan, add the onion, the tomatillos, the whole tomatoes and garlic and saute them all together until the onions are golden brown. Then remove from heat and skin them.
Put all the ground spices and the roasted vegetables into a blender, add a little chicken stock and blend until smooth.
Pour the blended mixture into a large pot and add the remaining chicken stock to it.
To improve the consistency of the mixture, add the cornflour to two tablespoons of chicken stock and pour it into the pot with gentle stirring. Continue stirring until it reaches the desired consistency.
Finally add the chicken and the vegetables to the pan and simmer the stew on low heat for another 30 minutes until it is completely cooked and reaches its rich and thick consistency. Garnish with coriander leaves.