Place the chicken in a pot with just enough water to cover. Sprinkle with salt, and add the peppercorns, bay leaves, quartered onion and herb bundle. Bring to a low boil, and then reduce the heat and poach the chicken until cooked through, 30 to 40 minutes.
Remove the chicken, straining the stock, and then wipe the pot clean and return to the heat. Peel the skin off the chicken breasts and cover to keep warm until the sauce is ready. Dry toast the pumpkin seeds over medium heat until puffed and toasted, 4 to 5 minutes. Reserve 1/4 cup of the toasted seeds for garnish. Puree the remaining seeds in a blender or food processor with 1 cup of the stock, the chopped onion, EVOO, cumin, epazote, if using, cinnamon, tomatillos, lettuce, garlic and chiles.
Transfer the pumpkin seed puree to the pot with 2 1/2 to 3 cups of the reserved chicken stock and simmer to develop flavor and thicken, 20 minutes. Adjust the seasonings.
Pour the sauce over the chicken and garnish reserved toasted pumpkin seeds. Serve with warm corn tortillas, for wrapping, and minced red onion, for topping. If you would like to serve with a vegetable, steamed or sauteed zucchini is perfect.
For a make-ahead meal, simmer the sauce only until not quite thick enough to serve, 10 minutes. Return the chicken to the pot, turn off heat and let cool before covering and storing. Reheat over medium heat and thicken the sauce, to serve.