combine seasoning mix in small bowl. peel and parboil the tomatillos. dropping them into 2 quarts boiling water. When it returns to boil, reduce heat and simmer for 10 minutes. drain and set aside. sprinkle chicken with 2 tablespoons seasoning mix. rub in well. heat the oil in a 5 quart pot over high heat, just until oil begins to smoke about 4 minutes. brown the chicken in batches, make sure you brown thoroughly 3 minutes per side. remove from pan and set aside. add mushrooms and onions and remaining seasoning mix to the pot, cook about 3 minutes. then add tomatillos, reduce heat to medium, cover and cook for 5 minutes. mash tomatillos with a spatula. stir in the stock, return the chicken and accumulated juices to the pot, bring to a boil, reduce heat to low, recover and simmer for 30 minutes. serve immediately.