In a dry cast-iron skillet toast pumpkin seeds, cumin seeds, pepper and oregano over a low heat, shaking the pan occasionally. Heat until aromas are released, about 5 minutes. Remove before seeds begin to brown and set aside to cool. In a blender grind mixture until a powder forms. In blender or food processor puree tomatillos, serrano chile, garlic, lettuce, cinnamon, cilantro, radish tops, onion and salt until smooth. Preheat oven to 350 degrees. In a large saucepan heat olive oil over a high heat. Carefully, pour in pureed tomatillo mixture, and let sizzle for 30 seconds. Stir in stock. Reduce heat and simmer for 10 minutes. Turn up the heat to high and bring to a boil. Stir in ground nut mixture and remove from the heat. Puree in a food processor or blender, in batches, if necessary. Pass through a fine strainer, pressing with the back of a ladle to extract all the juices, into a 13 x 10-inch flameproof baking dish. Place the chicken breasts in the sauce and bring to a boil over moderate heat. Cover with foil and bake for 20 minutes. Serve the chicken and sauce over a bed of white rice. Sprinkle radish slices over the top.