Put whole chicken in a casser#le, cover with the stock, and bring to a simmer. Cover pot and poach chicken just below the simmering point over very low heat or in a very low oven (275 degrees) for 1 hour. Remove chicken from stock and let it cool enough to carve it. Toast the pumpkin seeds together with the peppercorns and cumin in a hot ungreased skillet about 5 minutes, stirring constantly to prevent scorching. Put into a blender and grind fine. Remove outer husks from the tomatillos, put them in a pan with cold water to cover, and bring to a simmer. Simmer 10 minutes, drain, then add them to the blender with 1/2 cup of their liquid and blend to a smooth thick puree. Chop all the remaining ingredients and add them to the blender. Add chicken stock, if needed, to thin the puree. Pour sauce into the skillet and simmer about 10 minutes to evaporate some of the liquid. Carve chicken in 4 or more pieces and add to the sauce, coating the pieces well. If possible,let chicken sit overnight in the sauce so that the seasoning permeates and the hotness of the chili diminishes. Reheat gently so that sauce does not boil. Serve hot or at room temperature.