Toast the pumpkin seeds in a dry frying pan, shaking pan or stirring constantly until they are browned and toasty (for pine nuts) or until they have browned and popped (for pumpkin seeds). Remove from the heat and transfer to a blender jar if you're using pine nuts or to a bowl if you're using pumpkin seeds. If you're using pine nuts, blend with 1 cup of the stock until smooth, then strain through a medium-mesh strainer. If you're using pumpkin sees, pulverize in a spice mill in batches and sift into a bowl. Stir in 1 cup of the stock and mix well. Set aside. Simmer tomatillos in water to cover with the chilies (remove stems, seeds and membranes from chilies) and simmer for 10 to 15 minutes. Drain and place in the blender jar. Add the onion, garlic, and cilantro. Grind the spices in a spice mill and add to the blender jar. Add the lettuce leaves and salt. Blend until the mixture is smooth. Heat the oil in a heavy-bottomed saucepan or casserole over medium heat. Add the nut or seed mixture and cook, stirring constantly, until the mixture thickens, about 5 minutes. Add the tomatillo mixture and cook again, stirring for about 5 to 10 minutes, until the mixture is thick. Stir in the remaining 2 cups chicken stock, bring to a simmer, cover partially, and simmer for 30 minutes. Taste and correct seasonings. If the mixture seems too thick, stir in a little more chicken stock. Yield: About 3 cups sauce. Serve with grilled or poached chicken breasts as main course. Use as a dip with crudites, as a topping for nachos or enchiladas. Add lime or lemon juice or vinegar and use as a salad dressing. Sauce freezes well. ~-